WASHINGTON — After a surprising string of victories last fall, Democrats now face a challenging terrain as they look to hold onto their Senate majority in 2014 and prevent Republicans from gaining full control of Congress during President Barack Obama's final two years. His party must defend a hefty 21 seats, including seven in largely rural states that the president lost last fall.

The task of maintaining control of the Senate has grown even more daunting in recent weeks, with four Senate Democrats announcing plans to retire. They are Tom Harkin of Iowa, Jay Rockefeller of West Virginia, Frank Lautenberg of New Jersey and Carl Levin of Michigan, who disclosed his plans Thursday. A fifth Democratic retirement could come soon from Tim Johnson of South Dakota.

Democrats control 55 seats in the Senate, after November elections in which they did better than expected and gained two seats to pad their majority. That means Republicans would need to pick up six seats next year to take control for the first time since 2006.

Twenty months before the mid-term elections, Republicans are laying the groundwork to try to capitalize on the defense-playing Democrats, working to recruit strong candidates in Arkansas, Alaska, Louisiana, North Carolina, South Dakota and West Virginia — all states carried by Republican presidential nominee Mitt Romney last year. They're also buoyed by history, which shows the party controlling the White House typically loses seats during the midterm of a second-term president.

"The map looks pretty good" for the GOP, said Greg Strimple, an Idaho-based Republican pollster for Senate and gubernatorial candidates. "If I had a deck of cards to play, I'd rather play the Republican deck than the Democratic deck."

Indeed, Republicans have only 14 of their seats up for re-election and only one — Sen. Susan Collins of Maine — is in a state Obama carried last year. Just two GOP senators have said they will retire — Mike Johanns of Nebraska and Saxby Chambliss of Georgia — and they represent states that favor Republicans.

Democrats say 2014 could be a repeat of the past two election years, when their well-funded candidates benefited from the missteps of tea party Republicans who were nominated in bruising primaries over more mainstream GOP candidates.

Mindful of those scars, Republicans are watching to see if such polarizing primaries materialize in states like Georgia, Michigan, Iowa and South Dakota. The outcome of those primaries could determine whether the GOP will try to take advantage of Democratic retirements.

Jim Manley, a former aide to Senate Majority Leader Harry Reid, D-Nev., doesn't deny that the spate of Democratic retirements make it that much tougher to keep control in 2014. "The math is very much against Democrats," he said. Even so, he adds, "The real question, however, is whether Republicans are going to keep on nominating extremists or they're going to finally figure out that they've got to go mainstream."

At this early stage, both sides are focusing mostly on recruiting candidates — and watching for signs of how the opposition is positioning.

An early skirmish has emerged in Kentucky, where Senate Republican leader Mitch McConnell faces re-election next year and is working to prevent both Republican and Democratic challenges. Among the Democrats talking about running: actress Ashley Judd, who grew up in Kentucky but lives in Tennessee.