Final result: CNN projects Tester wins

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. Both campaigns went negative in the fight over Social Security, and both said the other is distorting their views.

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

Final result: Deb Fischer wins

Republicans argued that Kerrey, once a popular governor and two-term senator, lost touch with the conservative state after spending years living in liberal New York City, where he worked as president of the New School. The conservative super PAC American Crossroads began running ads against Kerrey even before he launched his campaign.

For his part, Kerrey tried to overcome the state's GOP tilt in a presidential election year by stressing his commitment to entitlement reform, a balanced budget, and other conservative fiscal issues. He also won a surprise late endorsement from former Nebraska Republican Sen. Chuck Hagel, considered by many to be a GOP renegade.

Republicans began eyeing Nelson's seat even before he announced his retirement. Fischer surprised GOP leaders by squeezing out a primary win over two opponents considered stronger at the start of the year.

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

Final result: Dean Heller wins

Democrats' strength in Nevada was put to the test once again in this Senate election. With voter registration numbers on their side, seven-term Democratic Rep. Shelley Berkley had 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 recent polls. Heller's strong fund-raising ability also helped him keep the advantage in a state with a heavy Latino population that normally overwhelmingly votes Democratic.

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

CNN projects Heitkamp wins

Former state Attorney General Heidi Heitkamp gave Democrats their best opportunity to keep the seat of retiring five-term Sen. Kent Conrad. Republicans hoped first-term Rep. Rick Berg will repeat the party's success of 2010, when they took over retiring Democrat Byron Dorgan's seat.

The state with the lowest unemployment in the country has a history of split-ticket voting, giving Heitkamp an opportunity to show her independence from national Democrats. She's publicly disagreed with Obama on issues like energy, which she points to as evidence she won't be a rubber stamp. She enjoyed high favorables and solid support from Republican ticket-splitters and has hammered Berg for his connection to a controversial real estate company.

Ohio: Sen. Sherrod Brown (D) vs. Josh Mandel (R)

Final result: Sen. Sherrod Brown wins

Primarily because of those frequent trips from President Barack Obama and Republican candidate Mitt Romney, the contest between Brown and Mandel played out in Ohio against the backdrop of the presidential election.

Fueled by Super PAC money and advertising, the race got nasty at times. In total, $67 million dollars was spent in the election, according to the Center for Responsive Politics. Much of that -- $34 million -- was sent by outside groups advocacy groups.

At the three debate, Mandel and Brown battled over the auto bailout, abortion and each other's record. The testiness of the race spilled out on the debate stage -- at one debate Mandel told Brown to "calm down" and called him "a liar."

Brown was first elected to the Senate in 2006, during a wave of Democratic victories in the Senate and House. Throughout most of the 2012 race, especially in September and October, Brown led Mandel by around 10 percentage points in independent poling.

Pennsylvania: Sen. Bob Casey (D) vs. Tom Smith (R)

Final result: Sen. Bob Casey wins

Though recent polls gave Republicans hope where they previously had little, freshman Democratic Sen. Bob Casey of Pennsylvania was re-elected Tuesday.

GOP candidate Tom Smith, a wealthy business executive, lagged far behind Casey for much of the race.