Races are tight, but GOP looks to seize control of the Senate

On Tuesday, millions of Americans will head to the polls to vote in the most costly midterm elections ever. Here's what you need to know.

Late-in-the-game polling is predicting a strong night for the GOP on Tuesday, as the party appears poised to regain control of the Senate and make gains in the House of Representatives. But many races remain extremely tight, and candidates of both parties are on a last-minute campaign blitz as they aim to mobilize the vote and swing races in their direction.

To secure control of the upper chamber, Republicans need to pick up six seats—seven if Kansas’ wild-card independent wins and chooses to caucus with the Democrats as he's hinted he might. Three states currently held by Democrats -- Montana, West Virginia and South Dakota -- are likely to be pickups for the GOP, leaving three or four more seats the party needs to win control.

The fate of the Senate will be decided by close races in these states: Alaska, Arkansas, Louisiana, Colorado, Georgia, Iowa, New Hampshire, Kentucky, Kansas and North Carolina.

In Kentucky, polls suggest incumbent Senate Minority Leader Mitch McConnell has pulled ahead of Alison Lundergan Grimes, the young Democrat championed by the Clintons. In a recent NBC/Marist survey, McConnell leads Grimes by nine points among likely voters, 50% to 41%. A win for McConnell, a 30-year Senate veteran, would position him to become Senate majority leader next year if his party does as well as expected. 

Related: McConnell campaign event proves sleepy, underwhelming 

In Arkansas, Republican Tom Cotton is leading Democrat incumbent Sen. Mark Pryor. The gap has been closing: An NBC News/Wall Street Journal poll late last month showed Cotton leading by just two points; a month earlier, he lead by five. Still, it’s an upward climb for the Democrat whose president is hugely unpopular—just 34% of Arkansas voters say they approve of the job Obama is doing. Cotton has hit Pryor hard, both over his vote for Obamacare and in general on the president's unpopular agenda.

In Iowa, the Des Moines Register’s final poll of the election saw Republican Joni Ernst polling seven points ahead of Democrat Bruce Braley, even as Braley and other Democrats argue that the poll is inaccurate. “Neither side believes that,” Democrat Sen. Tom Harkin said at a rally for Braley. “I just don’t buy the poll because we haven’t had an open seat in 40 years.” Ernst's folksy, Sarah Palin-esque demeanor has captured many voters in the Democratic-leaning state, while Braley has struggled to recover from a handful of gaffes, particularly his condescending remark that the state's senior Sen. Chuck Grassley was just "a farmer from Iowa without a law degree."

Related: Iowa Democrats: The polls are wrong

In Louisiana, incumbent Democrat Sen. Mary Landrieu has been running behind, but this race is likely to go to a run-off if none of the three candidates earn a majority on Tuesday. Landrieu has been dragged down by President Obama's low popularity ratings in the state, and Landrieu has sought to ally herself to a more popular Democrat, Hillary Clinton.

In Georgia, Democrat Michelle Nunn is in a tight race with Republican David Perdue. This race may also end up in run-off if neither candidate earns 50% of the vote, since Libertarian Amanda Swafford is likely to pick up a small share for herself. Perdue, a millionaire businessman, has been criticized for his support of outsourcing jobs overseas, while Nunn, like other red-state Democrats, has been hit as a "rubber stamp" to Obama.

Related: Democratic hopes run high in Georgia's toss-up Senate race

In Alaska, polling is historically unreliable, but incumbent Democrat Sen. Mark Begich and Republican Dan Sullivan are dueling in an undeniably tight race. According to a RealClearPolitics average of all the polls taken, Sullivan is in the lead. Still, it may be weeks until we know: In 2008, it took two weeks for ballots—many of them absentee--to be counted. Sullivan has painted himself as a conservative family man and has hit Begich for his connection to Washington's Democratic elite, the economy, and Obamacare. Begich has hit Sullivan for his opposition to abortion and championed his mastery of local issues.

In Colorado, the state where President Obama accepted his party’s nomination for president six years ago, incumbent Democratic Sen. Mark Udall is struggling to hold his seat in one of the country’s tightest races. Most surveys have shown Republican Rep. Cory Gardner polling a few points ahead of Udall. Women's issues have dominated Colorado's race, with Udall betting big on Gardner's opposition to abortion would help produce a large enough margin among women voters for Udall to eke out a win. 

In New Hampshire, Democrats are counting on incumbent Sen. Jeanne Shaheen to hold off Republican Scott Brown. Shaheen has portrayed Brown, a former Massachusetts senator, as a carpetbagger, while Brown has attacked her as being a vote for Obama's policies. 

In Kansas, Independent Greg Orman has been running even with or ahead of Republican Pat Roberts, a surprising turn of events in one of the nation's reddest states. Orman hasn’t said which party he’ll caucus with if he wins and has suggested he could switch back and forth between parties. 

In North Carolina, Sen. Kay Hagan has been polling just slightly ahead of Republican Thom Tillis for weeks; she’ll need a strong turnout from her base to hold her seat—if early voting is up significantly in the state, with 1.1 million ballots already cast, she may be in luck. Tillis, the state House speaker, has been hit hard over his opposition to abortion rights and the hard-right direction of the state legislature, while Hagan has been on the defensive for her vote for Obamacare and the general state of the economy.