Kyle Kondik is managing editor of Sabato’s Crystal Ball, a nonpartisan political newsletter produced by the University of Virginia Center for Politics. He also directs the center’s Washington, D.C., office.

If Republicans capture control of the U.S. Senate, there will be many explanations for their victory: President Obama’s poor numbers, a great Senate map filled with attractive opportunities, a generally strong slate of candidates, the success of establishment-backed Republicans in primaries and others.

But one of the biggest factors will have hardly anything to do with the national political climate or, really, the campaign as a whole. Five Democrats, all of whom are old enough to be eligible for Medicare, decided not to run for another term in the Senate. Their decisions, all announced before May 2013, are a huge but largely forgotten boon to GOP hopes.

Story Continued Below

The five retirements were: Max Baucus of Montana, Tom Harkin of Iowa, Tim Johnson of South Dakota, Carl Levin of Michigan and Jay Rockefeller of West Virginia. (A sixth, Frank Lautenberg of New Jersey, had announced his retirement, but he later died: Sen. Cory Booker, a Democrat, now holds the seat after a special election last year.)

Retirements present a challenge for the incumbent party because it’s easier for a party to hold a Senate seat when an incumbent runs for reelection. Significantly easier.

Over the past half-century, about 85 percent of incumbent senators running in the general election were reelected. In open seats, during the same time period, the incumbent party held the seat just about 60 percent of the time. So not having an incumbent in a Senate race substantially reduces the odds of victory.

The importance of open seats to GOP Senate hopes is particularly pronounced because of the party’s recent inability to defeat Democratic incumbents.

In 1980, Republicans beat an eye-popping nine Democratic incumbents on Election Day to capture control of the Senate for the first time in a quarter-century. Since then, the GOP has not defeated more than two Democratic Senate incumbents in any general election. The party’s best recent Senate years, 1994 and 2010, were built largely on winning open seats (six in 1994, and four in 2010). Democrats, meanwhile, have had more success: They beat seven and six incumbent Republicans, respectively, in recapturing the upper chamber in 1986 and 2006. The latter year, 2006, was notable in that Democrats did not capture a single open seat in netting the six seats they needed to eke out a narrow 51-49 Senate edge.

Republicans seem likely to beat more than two Democratic Senate incumbents this November for the first time in almost 35 years: Sens. Mark Begich of Alaska, Mary Landrieu of Louisiana and Mark Pryor of Arkansas all have less than 50-50 odds of winning, according to the University of Virginia Center for Politics’ Crystal Ball ratings, which I help formulate. Sen. Mark Udall of Colorado is right at 50-50, and Sens. Kay Hagan of North Carolina and Jeanne Shaheen of New Hampshire are both in tough races too.

But in all likelihood, the foundation for this year’s GOP gains will be built on winning open seats. That’s why the Democratic exits loom so large in the upcoming election.

An election where these five senators chose to run for another term would look significantly different: There would probably be more competitive races, and Democrats would have a greater number of redoubts to hold off the Republican advance.

Let’s assume, for the purposes of this “what if” exercise, that all five retirees were healthy enough and eager enough to have run for reelection, which is of course a big assumption for a group whose average age is 74 years old:

West Virginia: Rockefeller might still be an underdog to Rep. Shelley Moore Capito (R), who entered the race before the incumbent retired. But that race would have been much closer, presumably, than Capito’s largely sleepy contest against West Virginia Secretary of State Natalie Tennant, and the National Republican Senatorial Committee would not have been able to just take a pass on the race, as it has this cycle. Capito’s strong performance as a candidate is an often overlooked bright spot for Republicans this cycle – she’s done so well, in a state where the GOP hasn’t won a Senate seat in more than a half century, that the race never became obviously competitive – but running in an open seat has made her job easier. The Crystal Ball rating here is Safe Republican; with Rockefeller in the race, we’d probably rate it just Leans Republican or maybe Toss-up. A few polls before Rockefeller retired suggested Capito would have started the race with a narrow lead, and Rockefeller did himself no favors by not being 100 percent pro-coal in recent years, a political problem in a state where coal is still king.

Montana: Baucus, like Rockefeller, would have been in for a very tough race in 2014 if he had run. The Democratic polling firm Public Policy Polling found him trailing 49 percent to 44 percent in early 2013 against Rep. Steve Daines, who eventually became the GOP nominee. Baucus announced his retirement in April 2013, and then later resigned to become ambassador to China. Gov. Steve Bullock appointed then-Lt. Gov. John Walsh, a fellow Democrat, to fill the vacancy. Walsh’s campaign, of course, fell apart over the summer under the weight of plagiarism. Now Democrats are stuck with little-known nominee Amanda Curtis, a state representative, and Daines is a huge favorite.