Among the 33 Senate races this election, there are 10 that will determine which political party will have control in the next Congress.
The Senate currently has 51 Democrats, two independents that caucus with the Democrats, and 47 Republicans. For the Senate to change control, Republicans need a net gain of four seats if President Barack Obama is re-elected, or a net gain of three seats if Mitt Romney wins the presidency (because the vice president breaks the tie).
The 10 contested Senate races include Arizona, Connecticut, Indiana, Massachusetts, Missouri, Montana, Nevada, Ohio, Virginia, and Wisconsin.
This year was supposed to be a good year for Republicans to make gains in the Senate, but due to some retirements and some gaffes by two Republican candidates, the prospect of a Republican-controlled Senate in 2013 appears more in doubt.
In Maine, Republicans have likely lost at least one seat due to the retirement of Sen. Olympia Snowe. Angus King currently has a comfortable lead there. Though he is an independent and has not said which party he will caucus with, most observers believe he will caucus with the Democrats.
In Arizona, the retirement of Sen. John Kyl has left his seat in a close contest between Rep. Jeff Flake (R) and former Surgeon General Richard Carmona (D).
Two Republican incumbents are also in tight races: Scott Brown in Massachusetts and Dean Heller in Nevada.
Brown won a special election after Ted Kennedy's death in 2011 left his seat open. The Republican was able to win in that heavily Democratic state by pulling together Republican and independent voters in a low-turnout election. This year's election, though, will not be low-turnout. His challenger, Elizabeth Warren, only needs to rally her Democratic base and pull a few independents to her side to win.
In Nevada, which is also a battleground state in the presidential contest, Democrats are advantaged with a strong party organization, organized labor and a growing Latino community. With one of the worst economies in the country, Nevada voters, though, may be eager for change. But in the state with a reputation for split-ticket voting, change could mean voting against their Republican senator and against the Democratic president.
In two of the states where Republicans had hoped for easy pick-ups, their nominees both used poor word choices when talking about rape (see here and here). In Missouri, Todd Akin (R) is now in an uphill battle to unseat Claire McCaskill (D). And in Indiana, Richard Mourdock (R) is trying to keep that seat in Republican hands against Joe Donnelly (D).
Democratic incumbents are trying to hold onto their seats in Montana and Ohio.
First term Senator John Tester won election in the strongly conservative Montana in 2006 due to the anti-Republican mood of the electorate that year. He has mostly voted as a conservative Democrat, but his vote in favor of the Affordable Care Act, or "Obamacare," has put him in a tight race.
Ohio, the swingiest of swing states in the presidential race, shows a tight race between Sen. Sherrod Brown (D) and Josh Mandel (R), who, as state treasurer, has already won a statewide election.
Democrats are trying to hang onto three open seats in Connecticut, Virginia and Wisconsin.
In the Connecticut race to replace independent Joe Lieberman, Linda McMahon (R), CEO of the WWE entertainment wrestling company, is distancing herself from the national Republican ticket and running as a liberal on social issues. Once thought to be a solid Democratic seat, her race against Rep. Chris Murphy (D) is now within the margin of error.
In Virginia, a swing state in the presidential race, retiring Sen. Jim Webb would probably be in a tight race if he had chosen to run again. The race now has two candidates who are already well known in the state: former Governor Tim Kaine (D) and former Senator George Allen (R).
Wisconsin has traditionally been a Democratic-leaning state. In 2010, though, it elected a Republican governor (Scott Walker) and senator (Ron Johnson), and attempts to recall Walker and repeal one of his signature legislative accomplishments failed. Now, Wisconsin has become a swing state in the presidential race and its senate race is a close contest between former Governor Tommy Thompson (R) and Congresswoman Tammy Baldwin (D).