Democrats are convinced King will side with them. As evidence, they point to King's support for Obama and many of his policies. In a sign of that confidence, national Democrats ignored the Democratic candidate Dill and spent heavily against Summers in order to help King.

Republicans also believe that King will caucus with Democrats, so they spent millions to defeat him.

But an aide told CNN last week that King still has not made up his mind and won't until he gets to Washington to see the state of play.

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

CNN projection: Elizabeth Warren wins

In a key pick-up of a Republican Senate seat, Warren defeated Brown Tuesday, according to a CNN projection.

Warren, a Harvard University law professor and former Obama administration financial consumer advocate, beat the freshman senator who was seeking a full term to the seat he won two years after the death of Democratic senator Ted Kennedy. Brown, the first GOP Senator from deep blue Massachusetts in decades, billed himself as an independent who voted for the needs of Massachusetts, not the demands of either party.

The Warren-Brown race was one of the costliest and nastiest campaigns this cycle. It was marked by Brown's repeated accusations that Warren claimed to be a minority in order to advance her career. She denied that was her motive for listing her ancestry as Native American, saying she was told about her heritage by her family.

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 was able to crack 50% in the polls. Both campaigns went negative in the fight over Social Security, and both said the other is distorting their views.

Like the Massachusetts race, there were few undecideds left, except here Republicans were hoping that worked in their favor.

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 was being put to the test once again in a Senate election. With voter registration numbers on their side, seven-term Democratic Rep. Shelley Berkley hoped 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 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. 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)

Open seat -- Sen. Kent Conrad (D) is retiring