Democrats think their candidate's strengths and the state's growing Hispanic population will lead to their party's first successful Senate election since 1988. But Republicans point to no significant changes in Hispanic voting records, Flake's endorsements from Sens. Kyl and John McCain, and the state's traditional GOP support as reasons for a Flake victory.

Connecticut: Rep. Chris Murphy (D) vs. Linda McMahon (R)

Open seat -- Sen. Joseph Lieberman (I) is retiring

After waging a competitive but ultimately unsuccessful bid for U.S. Senate in 2010, former wrestling executive Linda McMahon is making a strong comeback this year. Buoyed by the millions of her own money she invested in the campaign, McMahon's made an effort to soften her image with ads about her personal life and combat attacks from third-term Rep. Chris Murphy.

Despite McMahon's significant financial advantage, Murphy is polling even or ahead of his opponent. In a state where President Barack Obama won by more than 20 points in 2008, a tie in the polls going into Election Day could mean a Murphy win on the president's coattails.

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)