McCain debates Kirkpatrick on Oct. 10 in Phoenix. (Ross D. Franklin/AP)

Sen. John McCain (R) is poised to win a sixth term after a tough two-front battle with his state’s far-right wing and Democrats accusing him of flipping his positions to fit the moment.

McCain, 80, first had to get through a primary against Kelli Ward, who ran as a tea party insurgent proudly backing Donald Trump and aped Trump’s criticism of McCain over veterans’ legislation. McCain won comfortably, but received a tad more than 50 percent of the GOP vote.

That set up a quick and potentially difficult general-election battle against Rep. Ann Kirkpatrick (D), who represents the state’s largest district. But McCain quickly galvanized most of the Republican vote, running as a check against a possible Hillary Clinton presidency and trying to steer clear of discussing Trump.

Kirkpatrick has aired tough ads showing McCain’s changing positions, including in 2010 calling for a “danged fence” along the Mexican border. But Washington Democrats took a pass on engaging the race. Both the Democratic Senatorial Campaign Committee and its super PAC ally, Majority PAC, have not devoted resources to the race.

McCain formally dumped Trump in early October. A CNN poll showed him with a 13-point edge in the past week.

— Paul Kane

Harris gives a thumbs up to supporters during her visit to the campaign office of Rep. Ami Bera in Elk Grove, Calif., on Nov. 3. (Rich Pedroncelli/AP)

When Sen. Barbara Boxer (D) announced her retirement, California Attorney General Kamala Harris became the instant front-runner. Other Democrats stepped back, and no strong Republicans came forward in the deep-blue state.

Enter Rep. Loretta Sanchez (D). Sanchez made it through the June primary, defeating several GOP candidates to finish a distant second to Harris. State election rules allow the top two finishers, regardless of party, to advance to the general election. But the Orange County congresswoman has struggled to catch up to Harris, who in recent polls was leading by more than 20 points.

Sanchez has campaigned as a moderate and she’d hoped to mobilize Latinos, California’s largest ethnic group. She has modest leads with Latino voters in polls, but Harris boasts double-digit margins with nearly every other demographic group.

Harris has the backing of the California Democratic Party, labor unions and progressive groups, and President Obama, who appeared in a campaign ad for her.

Sanchez, 56, would be the first Latina elected to the Senate. Harris, 52, whose parents emigrated from Jamaica and India, would become the second black woman elected to the Senate since 1992.

— Vanessa Williams

Rubio, right, and Murphy debate on Oct. 26 at Broward College in Davie, Fla. (Wilfredo Lee/AP)

The most important day in Florida’s Senate race was June 22, when GOP Sen. Marco Rubio announced that he would seek reelection — reversing a pledge he made when he began his ill-fated presidential campaign. Without Rubio, Republicans faced a messy primary featuring several unknown, untested candidates. Democrats, meanwhile, thought they had a winner in Rep. Patrick Murphy, a young up-and-comer who had triumphed in the nastiest and most expensive House race of 2012. Rubio’s reversal instantly gave the GOP an advantage, and they have not given it up.

Rubio and GOP allies have spent tens of millions of dollars attacking Murphy, trying to paint him as a spoiled rich kid with a questionable professional record. Murphy has fought back with ads capitalizing on Rubio’s presidential ambitions and his poor Senate attendance record while pursuing them. But Democrats pulled millions of dollars of ad reservations over the course of the fall as other races proved to be more competitive, leaving Murphy unable to match Rubio dollar for dollar.

Murphy hasn’t caught up in polling, but he’s not far behind. A late influx of Democratic spending and a stronger-than-expected Clinton win could spell an upset — and a devastating loss for Rubio.

— Mike DeBonis

Kirk, right, and Duckworth debate on Oct. 27 at the University of Illinois in Springfield. (Seth Perlman/AP)

Sen. Mark Kirk (R) has used a wheelchair since he had a stroke in 2012, and against virtually any other opponent, his return to the Senate would be a powerful narrative on which to run.

But his Democratic opponent, Rep. Tammy Duckworth, has a story just as powerful, having lost her legs in 2004 when her helicopter in Iraq was struck by an enemy rocket. It was during her recuperation that her home state’s two Democratic senators — Richard J. Durbin and Barack Obama — drafted her into politics. She has run a cautious but competent campaign withstanding attacks on her tenure as head of the state Department of Veterans Affairs and on her thin legislative record.

For Kirk, meanwhile, reelection was a tough bet, as Illinois last elected a Republican senator in a presidential year in 1972. And Kirk is his own worst enemy, talking his way into controversies that culminated in the most stunning congressional debate gaffe of 2016.

After Duckworth described her family’s military heritage, Kirk said he had “forgotten that your parents came all the way from Thailand to serve George Washington.” Although Duckworth’s mother is a Thai immigrant, her late father’s lineage can be traced to the Revolution.

— Mike DeBonis

Bayh participates in debate for Indiana's open U.S. Senate seat in Indianapolis on Oct. 18. (Michael Conroy/AP)

Young participates in debate for Indiana's open U.S. Senate seat in Indianapolis on Oct. 18. (Michael Conroy/AP)

Everything changed in Indiana’s Senate race when former senator Evan Bayh (D) announced on July 11 that he would be running to retake his seat. Until that day, Republicans were convinced that Rep. Todd C. Young (R) had an easy path to victory. Bayh entered the race with strong support from Democrats, who have fond memories of his time as governor and senator.

But Bayh proved ill-prepared for a punishing Senate contest and has been pummeled by accusations that he is a Washington insider who is out of touch with Hoosiers. First, he incorrectly listed the address of his Indianapolis condominium, then he battled reports that he spent the final days of his last Senate term interviewing for high-paying jobs.

A poll conducted Oct. 22 to Nov. 3 by WTHR-TV and Howey Politics shows Bayh trailing by five points.

Young, who served as a Marine, has made a name in the state as fresh-faced alternative to Bayh’s storied career. His campaign has benefited from a flood of outside support from groups such as the U.S. Chamber of Commerce and the Senate Leadership Fund, the Karl Rove-backed super PAC run by former aides to Senate Majority Leader Mitch McConnell (R-Ky.).

— Kelsey Snell

Kander speaks to supporters at a St. Louis union hall on Nov. 1. (Jim Salter/AP)

Blunt, left, and his wife, Abigail, campaigns in Missouri on Oct. 8. (David Weigel/The Washington Post)

Many Republican and Democratic operatives agree: No Senate challenger has run a better race than Jason Kander (D), Missouri’s secretary of state. It was a viral TV ad that had Kander assembling a military-style rifle with a blindfold that caught Washington’s eye, but it’s months of disciplined, pitch-perfect campaigning that puts him within a hair of unseating Roy Blunt (R) in a state likely to favor Donald Trump by double digits.

Kander, who was an Army intelligence officer, has leaned heavily on his military background and has found a receptive audience for his attacks on Blunt as a Washington insider.

Republican groups have been forced to come to Blunt’s rescue, financing ads linking Kander to Hillary Clinton and President Obama — some of them showing Kander’s face morphing into theirs.

Kander, meanwhile, has not passed up an opportunity to remind voters that Blunt lives in a D.C. mansion and that his wife and three of his children are lobbyists. And although Blunt has not distanced himself from Trump, he fits the profile of the D.C. denizen Trump is running against. Virtually every independent poll since September has shown a tied race or Blunt leading within the margin of error.

— Mike DeBonis

Cortez Masto shakes hands with Heck after an Oct. 14 debate in North Las Vegas. (Erik Verduzco/AP)

Senate Minority Leader Harry M. Reid’s retirement announcement rocked Nevada’s political scene, creating the GOP’s only chance to capture a Democratic seat on the Senate battleground.

The race pits Rep. Joe Heck (R), who represents the Las Vegas suburbs, against former state attorney general Catherine Cortez Masto (D), who would be the first Latina senator. The state is difficult to poll, both because of its new Hispanic population and because employees working in the Las Vegas gambling and nightlife industry are hard to reach.

Divergent polls show Heck leading by about seven points, or Cortez Masto ahead by the same margin. Neither candidate has broken through policy-wise, nor has either’s personality crashed through the din of commercials here.

Much of Heck’s campaign isn’t focused on his rival but instead on Reid — casting him as a villain who chose his puppet to succeed him. And Cortez Masto’s is focused on Donald Trump, trying to energize Hispanics and mocking Heck’s wavering support for the GOP nominee.

The race may be decided in “the rurals,” counties far away from the Las Vegas Strip where Trump is expected to do well. Heck has withdrawn his support for Trump and called for him to quit the race, which may be why he is underperforming there.

— Paul Kane

Ayotte, left, and Hassan during a Nov. 2 debate. (Jim Cole/AP)
New Hampshire

For months, polls in New Hampshire have shown Sen. Kelly Ayotte (R) in a near dead heat with Gov. Maggie Hassan (D). The latest Boston Globe poll, conducted at the end of October, showed Ayotte leading by 2.5 points, well within the margin of error.

The outcome of the closely watched race is expected to play a critical role in determining which party will hold the Senate majority next year.

Ayotte, a former state attorney general, has struggled to overcome a powerful downdraft created by the unpopularity of Donald Trump. She sank in the polls when struggling to respond to a leaked 2005 recording of Trump talking about kissing women and grabbing them without their consent — at first standing by Trump and eventually rescinding her support.

Hassan has benefited from running as a well-liked sitting governor but she has struggled to eke out a consistent lead. Hassan had to overcome her own gaffe when she struggled to answer whether she thinks Hillary Clinton is honest and trustworthy.

The race has attracted more than $89 million in spending from outside groups, making it the second most expensive Senate contest this year, according to the Center for Responsive Politics.

— Kelsey Snell

Strickland greets a poll worker before voting at the Franklin County Board of Elections on Oct. 27. (Jay Laprete/AP)

Portman tours Pioneer Pipe while campaigning in Marietta, Ohio, on Oct. 25. (Spencer Platt/Getty Images)

Sen. Rob Portman (R) was originally expected to be fighting for his political life in the campaign’s final days.

A consummate insider, having worked in both Bush administrations and as part of the House GOP leadership, he seemed ill-suited for the anti-Washington environment epitomizing this cycle.

But Portman seems to have prevailed after running what was probably the best race of any Republican candidate this year. He raised more money than all but one Senate Republican (Patrick J. Toomey, who is in a close contest), and spent it early and often, a smart tactic in hindsight.

His ads reintroducing voters to their first-term senator were effective, portraying his work to fight an opioid crisis with Senate legislation and to combat sex trafficking. Meanwhile, Portman’s negative ads battered Ted Strickland, who began the race as a well-liked former governor. But after being reminded of the job losses during Strickland’s term, voters had turned against the ex-governor by late August. National Democrats abandoned the race.

Of the 10 most recent public polls, all in October, Portman had a lead of at least 11 points. GOP strategists are already studying this campaign as a blueprint for future Republican Senate races.

— Paul Kane

Burr and Ross in a debate on Oct. 13. (Gerry Broome/AP)
North Carolina

Sen. Richard Burr (R) cuts an increasingly rare figure on Capitol Hill, a low-octane throwback to a time when the soul of the GOP was country-club conservatism, not sharp-edged populism. He has the old-school campaign ethos to match.

National Republicans fumed as he stuck with the winning formula from his previous two runs: Keep your powder dry until October, then hold on for dear life at the end. His opponent is Deborah Ross, a lawyer and former state lawmaker whose record as a civil-rights advocate many assumed would be too liberal for the state.

But Ross has proven a masterful fundraiser and fierce campaigner, and an avalanche of ads highlighting her past objections to the state’s sex-offender registry seem not to have deeply affected voters. Polls have shown a tight race for months, and although Burr has had a small lead in most recent polls, he has not been able to run far ahead of Donald Trump in a state where Democrats are making an extra-hard turnout push.

The final week was roiled when Burr was recorded in a private meeting cracking a joke about gun owners putting a “bullseye” on Hillary Clinton. Ross, meanwhile, has benefited from a pair of presidential visits.

— Mike DeBonis

McGinty and Toomey at a debate at Temple University in Philadelphia on Oct. 24. (Matt Rourke/AP)

The most expensive Senate race in history is in Pennsylvania, topping $140 million by late October, according to the Philadelphia Inquirer.

The money reflects the stakes of this contest. Democrats always believed their path to the majority probably required knocking off Sen. Patrick J. Toomey (R). Toomey tells supporters everywhere that their votes will keep the Senate in Republican hands.

Sen. Charles E. Schumer (N.Y.), the incoming Democratic leader, recruited neophyte Katie McGinty, persuading Vice President Biden to coax President Obama to endorse her in the primary.

Toomey and McGinty are basically deadlocked. McGinty has held a small lead in the past few polls — but over the past month, four showed the race tied and four gave Toomey a slight lead.

Toomey is performing well in the vote-rich Philadelphia suburbs. But he is doing less so in the rural stretch between Philadelphia and Pittsburgh, in part, strategists say, because he hasn’t said whom he supports for president. That may work near Philadelphia, where Hillary Clinton is crushing Donald Trump, but Trump is well-liked elsewhere.

Whichever party wins this race will have the edge to win the majority.

— Paul Kane

Johnson, left, and Feingold, right, meet in their second and final debate Oct. 18 in Milwaukee. (Mark Hoffman/AP)

For months, Democrats considered Wisconsin a near-certain pickup. It has been a reliably blue state in presidential years, one-term incumbent Ron Johnson (R) had not done much to raise his profile and well-known former senator Russell Feingold (D) had signed up for a rematch.

Summer polls showed Feingold with a double-digit lead, but the well-respected Marquette Law School Poll found last week that that margin had shrunk to a single point amid a late GOP ad blitz.

In the final week of the campaign, about $6 million was spent by campaigns, party committees, super PACs and outside groups — many of which had canceled earlier reservations.

Johnson, an Oshkosh plastics manufacturer, has sought to refresh his image as a no-nonsense outsider businessman, while casting Feingold as a career politician and a throwback. Feingold, meanwhile, has relentlessly driven a middle-class economic message focused on opposition to trade deals and tax loopholes, while tarring Johnson as an advocate for corporate interests.

Although the race has tightened, Democrats and some Republicans think Feingold has an edge: Early voting has outpaced expectations in Democratic strongholds, and Johnson is only now showing signs of outpacing Donald Trump.

— Mike DeBonis

The Democrats aren't likely to take back control of the House of Representatives on Election Day 2016, but they could see some wins. Here are some of the races where seats could flip. (Sarah Parnass/The Washington Post)