The Democrats’ ability to hold their Senate majority may rest on three candidates who haven’t run for office in more than a decade.

Former Maine governor Angus King speaks March 5 at Bowdoin College in Brunswick, Maine. King announced plans to run as an independent for the seat being vacated by Sen. Olympia Snowe (R-Maine), attempting a political comeback 14 years after he last ran for office. (AP Photo/Joel Page)

Democrats are also counting on a strong showing from former North Dakota Attorney General Heidi Heitkamp in that state’s open Senate race. The last campaign she ran was for governor in 2000, when she was diagnosed with breast cancer shortly before Election Day and lost by 10 points to now Sen. John Hoeven (R).

Taken together, King, Kerrey and Heitkamp are running in the top three races on this week’s Friday Line (see below), a reflection of the fact that their comeback campaigns matter — big time. If Kerrey and Heitkamp can seats for Democrats, which remains an unlikely prospect, their party will likely hold its majority, and if King wins and caucuses with Democrats, the party will have scored a key seat takeover and made things that much tougher on Republicans.

(For the purposes of this analysis, we’re considering King to be the Democratic candidate; though he’s not a member of the party, it’s assumed that he would caucus in the Senate with Democrats, who notably didn’t field a big-name candidate to oppose him.)

Republicans also have their comeback candidates.

Former Wisconsin governor Tommy Thompson (R) hasn’t run in Wisconsin since 1996. He did run for president in 2008, but if anything, his struggles to gain any kind of momentum in that campaign, despite his past as a popular four-term governor and cabinet official, are a testament to how difficult it can be to run campaigns more than a decade apart.

Sens. Richard Lugar (R-Ind.) and Orrin Hatch (R-Utah) aren’t technically running for the first time in a long time, but neither man has faced any real threat in the last three decades, and each faces a tough battle for his party’s nomination.

Sen. Dan Coats (R-Ind.) is the most recent example of a politician pulling a comeback in a major Senate race. Coats ran his last Senate reelection campaign in 1992, retired, and then came back in 2010 to run for his old seat — a gap of 18 years.

Early on, Coats struggled to raise money as fast as other big-name Senate candidates and looked as if he might struggle to win his primary. But by the end of the race, he had won easily:56 percent to 38 percent over then-congressman Brad Ellsworth (D).

“Campaigning is a lot like riding a bike: it all comes back rather quickly,” said a top Coats adviser in that campaign, Pete Seat. “In Coats’ case, he credits a competitive primary with helping ease him back into the process and, specifically, acclimating him with the many ways the 24-hour news cycle and Internet have changed politics.”

Coats himself reflected on the fast-changing nature of campaigns toward the end of the race in a great read from Indiana political expert Brian Howey.

“Everywhere I go, there is a camera on me recording,” Coats said. “It used to be you’d call a press conference, the press would show up and that was your message.”

The media environment is just one of many factors — including different contribution limits and a more nationalized electorate — that candidates are having to deal with these days.

How candidates like King, Kerrey and Heitkamp adjust will have a major impact in deciding who controls the Senate next year.

Now, without further ado, To the Line!

(A reminder that the races below are ordered according to likelihood that they will change parties, with No. 1 being the most likely.)

Off the Line: Ohio

On to the Line: Maine

10. Wisconsin (Democratic-held):The Republican field just keeps getting more crowded. The newest candidate is hedge fund manager Eric Hovde who spent the last two decades working in Washington, D.C. before moving back to the state over the summer. (Hovde has the same consulting team that guided another political newcomer in Wisconsin, now-Sen. Ron Johnson, to victory in 2010.) He joins
Thompson, former Rep. Mark Neumann and state Senate president Jeff Fitzgerald in the GOP primary. The race looks likely to get nasty quickly. Meanwhile, Rep. Tammy Baldwin has the Democratic primary to herself. (Previous ranking: 9)

9. New Mexico (D): The state may lean Democratic, but Republicans appear to have a good shot with former congresswoman Heather Wilson. In a recent automated Rasmussen poll, she was in a statistical tie with Rep. Martin Heinrich (D) and a dead heat with state Auditor Hector Balderas. But despite Lt. Gov. John Sanchez’s exit from the GOP primary (he also endorsed Wilson this week), the moderate Wilson isn’t getting a free pass to the nomination. (Previous ranking: 8)

8. Virginia (D): This may be the truest of the many toss-ups on this Line. In fact, when a recent poll showed former senator George Allen (R) leading by 8 percent, another poll soon showed former Democratic National Committee chairman Tim Kaine (D) leading by 9 points. The truth is probably right in between those two polls. (Previous ranking: 7)

7. Nevada (Republican-held): After a somewhat slow start to his 2012 race, appointed Sen. Dean Heller (R) has picked things up over the past few months. A Republican poll sponsored by the Retail Association of Nevada showed Heller at 47 percent to 44 percent for Rep. Shelley Berkley (D). Berkley is trying to gain traction by pressuring Heller to sign a pact banning outside groups from spending money on the race, but Heller dismissed the idea as a sideshow. (Previous ranking: 6)

6. Massachusetts (R): Just when it looked like Elizabeth Warren (D) had all the momentum in this race, three straight polls show Sen. Scott Brown (R) with a lead. In addition, Warren may have missed her chance at a big financial advantage when she agreed to a pact that excludes third-party groups from taking part in the contest. You can bet many left-leaning groups would want to back her. She’s still raising huge sums of money, though. (Previous ranking: 3)

5. Montana (D): This race is perhaps the most unchanged since we last did a Senate Line. Sen. Jon Tester (D) and Rep. Denny Rehberg (R) continue to focus on raising money while polling continues to suggest that the race between them is a nip and tuck affair. Assuming neither man makes any major mistakes, this race will come down to a simple question: Has Tester established enough of an independent identity with voters in the state to avoid being morphed into a President Obama clone by Republican ad makers? (Previous ranking:5)

4. Missouri (D): The GOP field here continues to be a mess, with state Auditor Tom Schweich the latest to have his name thrown into the mix. We have to believe that the more crowded the field gets, the better it is for Sarah Steelman, a grassroots candidate who may soon find herself the lone female hopeful in a four-candidate primary. No matter who wins, though, Sen. Claire McCaskill (D) is an underdog for a second term. (Previous ranking: 4)

3. North Dakota (D): Democrats believe that this is their sleeper race of the cycle, arguing that Heitkamp is underrated as a candidate and that Rep. Rick Berg (R) has major image problems in the state. (The hundreds of thousands of dollars he has already spent on bio ads so far in the race suggest Democrats might be on to something.) Still, this is North Dakota in a presidential year. While Obama kept his losing margin down in 2008, it’s almost certain he will lose by a more significant number this November. Can Heitkamp overperform the president by eight to 10 points? She’ll probably need to. (Previous ranking: 2)

2. Maine (R): The filing deadline passed here on Thursday, leaving the race nothing short of a jumbled mess. King is the favorite with no big-name Democrats filing, but Republicans could salvage retiring Sen. Olympia Snowe’s (R) seat if King and the Democratic nominee split up the vote enough. If he wins, King is most likely to caucus with Democrats, but he suggested recently that he might caucus with Republicans if they are in the majority. Cue intrigue. (Previous ranking: N/A)

1. Nebraska (D): Kerrey’s decision to run — after deciding not to run a few weeks earlier — gives Democrats a puncher’s chance in the Cornhusker State. And the Republican field — of which state Attorney General Jon Bruning remains the frontrunner — is not all that inspiring. But Nebraska is going to go for the Republican presidential nominee in a major way in November, and it’s going to be tough for any Democrat (including Kerrey) to win. (Previous ranking: 1)

Chris Cillizza contributed to this report.