This is a tough state for Republicans in federal office. The last Connecticut Republican to hold a U.S. Senate seat left office in 1989.

Indiana: Richard Mourdock (R) vs. Rep. Joe Donnelly (D)

Open seat -- Sen. Richard Lugar (R) was defeated in the primary

When six-term Republican Sen. Richard Lugar lost a bitter primary race to state treasurer Richard Mourdock, Democrats gained an unexpected opportunity to take over the seat with three-term Rep. Joe Donnelly.

Though Indiana is a Republican-leaning state, the polls have been close and some longtime Lugar supporters still haven't rallied behind the GOP nominee. Democrats pounded Mourdock for his comment at a debate two weeks before Election Day that "even when life begins in that horrible situation of rape, it is something that God intended." Mourdock the next day clarified his comment and added, "Rape is a horrible thing, and for anyone to twist my words otherwise is absurd and sick."

The full impact of the flap remains to be seen. Donnelly, a relatively conservative Democrat who opposes abortion rights, was one of few red-state Democrats to survive the Republican onslaught of 2010. He has kept the race competitive, but even with the Mourdock comments, Donnelly faces a Republican-friendly electorate and won't get much help from the top of the ticket.

Maine: Charlie Summers (R) vs. Cynthia Dill (D) vs. former Gov. Angus King (I)

CNN projection: Former Gov. Angus King wins

The unexpected retirement of three-term Sen. Olympia Snowe set the stage for a three-way contest between Democratic state senator Cynthia Dill, Republican Secretary of State Charlie Summers and former Gov. Angus King, an independent.

King, who quickly rose to frontrunner status, has declined to state until after the election which party he would caucus with in the Senate, though he is widely assumed to align with Democrats. The former governor has endorsements from the Human Rights Campaign, environmental groups and New York Mayor Michael Bloomberg's new super PAC, which supports gay rights, gun restrictions, and education reform.

The national Democratic party did not actively support Dill but instead ran ads hammering Summers. The national Republicans focused much of their ad fire on King, with the hope that he and Dill would split enough of the anti-Republican vote to ensure a Summers victory.

Massachusetts: Sen. Scott Brown (R) vs. Elizabeth Warren (D)

Massachusetts is perhaps the one race that has lived up to all its hype and more. As Republican Sen. Scott Brown competes for a full term against Harvard Law School professor Elizabeth Warren, the race for the state's few remaining undecided voters has taken a sharp tone. Brown launched harsh attacks questioning Warren's claims of Native-American heritage while also highlighting his own bipartisanship in the Senate.

Warren continues to attack Brown for protecting millionaires and for vowing to repeal Obamacare while highlighting her advocacy for the middle class and women's issues. Democrats hope the president's popularity and expected wide margin of victory in Romney's home state will counter Brown's popularity to make Warren the first woman to represent Massachusetts in the U.S. Senate.

Montana: Sen. Jon Tester (D) vs. Rep. Dennis Rehberg (R)

Democrat Jon Tester knew he'd have an uphill battle to a second term against six-term Rep. Denny Rehberg. Both are known quantities with high favorables in the state, though neither has been able to crack 50% in the polls. Both campaigns have gone negative in the fight over Social Security, and both say the other is distorting their views.

Like the Massachusetts race, there are few undecideds left, except here Republicans are hoping that works in their favor. Though the state has a history of voting for Democrats statewide along with a Republican for president, Barack Obama's disapproval ratings could drive the strong GOP turnout Rehberg needs to move up a chamber in Congress.

Nebraska: Former Sen. Bob Kerrey (D) vs. Deb Fischer (R)

Open seat -- Sen. Ben Nelson (D) is retiring

Republicans began eyeing two-term Democrat Ben Nelson's seat even before he announced his retirement. With Nelson out, Democrats pinned their hopes on Bob Kerrey, the former governor and two-term senator. Although a well-known figure in Nebraska, Kerrey spent most of his post-Senate career outside of the state, primarily in New York City where he served as president of The New School.

The conservative super PAC American Crossroads began running ads against Kerrey before he even declared his candidacy. The Republican nominee is state Rep. Deb Fischer, who scored a surprising win in a crowded GOP primary. Kerrey has an uphill battle to keep the seat blue; Fischer has been leading with at least 50% in both independent and partisan polls.

Nevada: Sen. Dean Heller (R) vs. Rep. Shelley Berkley (D)

Democrats' strength in Nevada is being put to the test once again in a Senate election. With voter registration numbers on their side, seven-term Democratic Rep. Shelley Berkley hopes to deny Republican Sen. Dean Heller a full Senate term (Heller was appointed to the seat after Republican John Ensign resigned).

A House ethics investigation and a lackluster debate performance have kept Berkley below Heller in the most recent polls. Heller's strong fundraising ability also helped him keep the advantage in a state with a heavy Latino population that overwhelmingly votes Democratic. Both parties are hoping for a win here, though the race should remain close through Election Day. Turnout for the presidential race could have the largest impact on the outcome of this race.

North Dakota: Heidi Heitkamp (D) vs. Rep. Rick Berg (R)