Jennifer Duffy, a Senate analyst at the nonpartisan Cook Political Report, put Republican chances of a Senate takeover at 40 percent but said that could jump past 50 percent by November.
"The Republicans are competitive in places I didn't think they would be, but they still have some challenges," she said, citing the potential for divisive Republican primaries in Georgia, North Carolina and Iowa that could leave the party with weakened, ultraconservative candidates who might have difficulty winning.
In Georgia, Nunn could face either Phil Gingrey or Paul Broun, Tea Party favorites in the House with a history of inflammatory comments on the campaign trail.
Democrats are hoping for a repeat of 2010 and 2012, when Tea Party candidates won Republican primaries but blew winnable races by committing gaffes that helped Democrats paint the entire party as outside the nation's political mainstream.
"One thing Democrats have going for them is the Republicans' continued ability to put their own foot in their mouth by making one provocative statement after the other," said Jim Manley, a former aide to Senate Democratic leader Harry Reid.
"As folks step toward the voting booth, they are going to think long and hard about giving control of the Senate to a bunch of Tea Party types," Manley said.
Some Republicans say the party has learned from those mistakes. They note that in Colorado, Gardner's entry into the race led two other prominent Republicans to bow out, a sign that party officials' effort to avoid debilitating primaries might be gaining ground.
"I'm not seeing the problem with primaries materialize the way it did in previous cycles," said Brian Walsh, a former aide at the National Republican Senatorial Committee. "Primaries are only a problem when you nominate someone who can't win in November."