Senate Races to Watch

That gives Democrats a better shot at winning the minimum five net seats they need to wrest from Republicans to win back control. If Democrats keep the White House, they only need to pick up four seats, since the vice president can break a 50-50 tie in the Senate. Here are 12 races to watch and the current outlook for each…:

ARIZONA: Republican Sen. John McCain, the GOP’s presidential nominee in 2008, entered his bid for a sixth term as the clear favorite against his Democratic opponent, Rep. Ann Kirkpatrick. The rise of Donald Trump and his anti-immigration views has served to tighten the race in Arizona, where Hispanics make up about 30% of the population and 22% of the state’s eligible voters. But the contest has consistently leaned in the incumbent’s favor. While Ms. Kirkpatrick has sought to tie Mr. McCain to the top of his party’s ticket, tensions between the senator and Mr. Trump have been evident throughout the campaign. Mr. Trump has diminished Mr. McCain’s record as a prisoner of war and the two have differing stances on immigration and major foreign-policy issues. Mr. McCain has sharply criticized Mr. Trump for arguing with the parents of a Muslim U.S. Army captain killed in Iraq. Then, in October, he split with Mr. Trump completely after the release of a 2005 video showing the celebrity candidate bragging about his lewd treatment of women.

COLORADO: Political insiders consider Democratic Sen. Michael Bennet one of the luckiest lawmakers of 2016. Emboldened by their defeat of incumbent Democratic Sen. Mark Udall in 2014, Republicans have long been eyeing Mr. Bennet’s seat in this swing state. But a crowded field of GOP candidates delayed the start of the general election against Mr. Bennet, who had more than $6 million in cash on hand, as of the end of June. Five Republicans qualified to square off in the state’s June 28 primary. The winner was El Paso County Commissioner Darryl Glenn, a tea-party backed conservative who had been supported by Texas Sen. Ted Cruz, popular among Republicans in this state.

FLORIDA: Republican Sen. Marco Rubio’s decision to run for re-election after pulling out of the presidential race boosts the GOP’s chances for holding this seat. Mr. Rubio, though, entered the race with the baggage of his own ambivalence toward the job. He missed numerous votes during his White House run, justifying it by saying the Senate had limited ability to set the nation’s agenda. His general election challenger is Democrat Patrick Murphy, a 33-year-old businessman who ran an arm of his family’s large construction business and whose father has contributed generously to support his campaign. Local news reports have raised questions about Mr. Murphy’s characterization of cleanup work in the Gulf of Mexico after the 2010 BP oil spill, allegations that Mr. Murphy says are coming from “right-wing groups.” Between that and Mr. Murphy’s portrayal of the incumbent as “not caring about his job,” the race is shaping up to be a brawl.

ILLINOIS: Republican Sen. Mark Kirk is perhaps the most endangered Senate incumbent this cycle. Elected in a Republican wave in 2010, he now faces a strong challenge from two-term Rep. Tammy Duckworth, an Iraq war veteran and former assistant secretary of the Veterans Affairs Department. Mr. Kirk, also a veteran, has run to the center in this race, calling for a vote on President Barack Obama’s nominee for the Supreme Court, Merrick Garland, for example, despite the Senate GOP leadership’s insistence on waiting until the next president is chosen. Ms. Duckworth has tied the incumbent to Donald Trump, though Mr. Kirk withdrew his endorsement of Mr. Trump on June 7. She has also touted her own endorsement from Mr. Obama—who held the Senate seat before being elected president in 2008.

INDIANA: The retirement of Republican Sen. Dan Coats gives Democrats a shot to win in Indiana, and the party is seeking to capitalize on the opportunity by fielding former Sen. Evan Bayh, who held the seat through 2010. Mr. Bayh jumped into the race late, supplanting the Democrats’ earlier pick, former Rep. Baron Hill. Mr. Bayh’s opponent is Republican Rep. Todd Young, who won his 9th-district seat in the 2010 Republican wave that gave the GOP control of the House. Mr. Young is an establishment conservative and Marine running on issues standard to his party—national security, veterans and his opposition to Obamacare. The Bayh name is an institution in Indiana—Mr. Bayh’s father Birch was a longtime senator and speaker of the state House. Mr. Bayh’s vulnerability, though, can be found in his own words—when he left the Senate, he said it was a dysfunctional body.

MISSOURI: Roy Blunt, Missouri’s junior senator, is considered fairly safe this year, but he faces a Democrat who has proven he can win statewide: Missouri Secretary of State Jason Kander. A 35-year-old Afghanistan war veteran, Mr. Kander could benefit from the year’s anti-establishment mood—and Mr. Blunt is the consummate insider. He served in the House for 14 years before winning his Senate seat in 2010, he’s a member of the GOP leadership and in 2015 became chairman of an appropriations committee that oversees the sprawling health and human services apparatus. But that status has also given him sway that he can say allows him to get things done. As a member of the Armed Services Committee, for example, he has worked to preserve funding for Navy orders of combat jets built by Boeing Co. in St. Louis.

NEVADA: The retirement of Senate Democratic Leader Harry Reid puts Nevada in play in what is likely to be one of the year’s most competitive races. His chosen candidate, Catherine Cortez Masto, is a former attorney general from a Las Vegas political family. Her opponent, moderate Republican Rep. Joe Heck, has controlled a congressional district that voted to elect a Democratic president in the last election and has an appealing backstory as a doctor and Army reservist. The election will test whether the strong political machine that Mr. Reid built can outlast him. Democrats maintain a registration edge in Nevada. Mr. Heck could also be stymied by an enthusiastic Latino voting base, especially with Donald Trump at the top of the ticket. Ms. Cortez Masto, though, has her own challenge as Republicans try to tarnish her as a puppet of Mr. Reid—but without his considerable Washington influence.

NEW HAMPSHIRE: First-term Republican Sen. Kelly Ayotte is popular in this state, but she has little margin for error in trying to beat back a challenge from Democrat Maggie Hassan, the state’s well-liked governor. Democrats outnumber Republicans in the state, and the Democratic Party has been energizing its base by tying Ms. Ayotte to presidential candidate Donald Trump, who won the New Hampshire primary but who is starting to engender buyer’s remorse among Republicans. The party has also been highlighting Ms. Ayotte’s decision to oppose hearings on Supreme Court nominee Merrick Garland. To fight back, Ms. Ayotte, who has a natural touch on the campaign trail, has been trying to focus on local issues and demonstrate bipartisanship in areas such as gun control. Before she faces off against Ms. Hassan, Ms. Ayotte first must win the GOP primary Sept. 13.

NORTH CAROLINA: This race wasn’t supposed to be close. Republican Sen. Richard Burr won re-election with 55% of the vote in 2010, and is a prodigious fundraiser who has built a reputation for constituent service. Democrats initially struggled to find a candidate but have since united behind Deborah Ross, a former state representative. But Republicans have been on the defensive over a new law that requires transgender people to use the public bathroom of the sex corresponding to their birth certificate. Democrats are trying to tie Mr. Burr to the bathroom law, and playing up his support of Donald Trump, while Republicans are focusing on Mr. Burr’s experience, which includes chairing the Senate intelligence committee. Republicans are also saying Ms. Ross is too liberal for North Carolina, as a former head of the state chapter of the state’s American Civil Liberties Union.

OHIO: Earlier this year, Republican Sen. Rob Portman was a major target of Democrats, who sensed the savvy legislator could be vulnerable in a year with Donald Trump at the top of the ticket. Since then, Mr. Portman has run one of the strongest Senate campaigns on the map, and has surged in the polls against his Democratic challenger, former Gov. Ted Strickland. His strength prompted the Democratic Senatorial Campaign Committee to cancel some advertising in the state, suggesting the Democrats saw better chances elsewhere. Mr. Portman has long had a financial advantage, raising more money than Mr. Strickland and picking up the support from well-funded Republican groups. He has also focused on local issues with an intensity that became a model for other candidates, focusing on his efforts to combat algae blooms in Lake Erie and even running advertisements in Ukrainian aimed at Ohio’s local Ukrainian population.

PENNSYLVANIA: Republican Sen. Pat Toomey’s biggest challenge in his fight against former state official Katie McGinty is fending off attacks by national Democrats linking him to Republican presidential nominee Donald Trump. Mr. Toomey has tried to keep distance between himself and Mr. Trump without totally disavowing the Republican candidate. Mr. Toomey is being helped by outside Republican-aligned groups who want the GOP to keep control of the Senate, and is also trying to focus on local issues. Ms. McGinty won a hard-fought primary with the help of Senate Democrats’ campaign arm and Emily’s List, which backs female candidates who support abortion rights. Both groups are expected to remain heavily involved in the matchup against Mr. Toomey. The fiscally conservative Mr. Toomey is looking to attract independent voters by highlighting some of his more centrist positions, including his 2013 support for expanding background checks for gun purchases.

WISCONSIN: Ron Johnson, one of the year’s most vulnerable Republican incumbents, is trying to hold on to his Senate seat by appealing to the state’s conservatives, a highly organized group that showed its muscle by blocking GOP presidential contender Donald Trump in the state’s GOP presidential primary. Mr. Johnson’s challenger, Democrat Russ Feingold, is an avowed liberal who held the seat for three terms before being ousted in the 2010 Republican wave. Mr. Johnson won his seat that year on a deficit-cutting message, but as austerity and debt have faded as political forces, he has broadened his focus to national-security issues, like pausing a program to resettle Syrian refugees. Mr. Feingold is calculating that his message on making college affordable and support for the Affordable Care Act, will be more appealing in a presidential election year, when more Democrats show up at the polls.

Written by Kristina Peterson, Arian Campo-Flores, Siobhan Hughes, Susan Benkelman and Valerie Bauerlein. Published August 11, 2016 at 5:30 a.m. ET | Updated October 5, 2016 at 2:25 p.m. ET.  Reprinted here for educational purposes only. May not be reproduced on other websites without permission from The Wall Street Journal. Visit the website at wsj .com.