Prospects may have improved for Mitt Romney after a strong first debate performance, but election handicappers say Republicans are still looking at an uphill climb to win control of the Senate — and thus, Congress — come November.

Senate Democrats dominate the chamber by a narrow but significant margin of 53-47, meaning Republicans would need a net gain of four seats to flip the balance of power. While a total of 33 seats are on the ballot this year, the battle for a majority hinges on fewer than a dozen key races.

The toss-ups include obvious presidential swing states like Virginia, where former Gov. George Allen, a Republican, is vying with former Democratic Gov. Tim Kaine for his old seat, and Wisconsin, where former Republican Gov. Tommy Thompson is running against Rep. Tammy Baldwin, a Democrat.

But they also include a few surprises as both parties eye pick-up opportunities in otherwise hostile territory.

Connecticut is a prime example. The retirement of longtime Sen. Joe Lieberman, an independent who caucuses with the Democrats, has given rise to a brutal face-off between Democratic Rep. Chris Murphy and Republican Linda McMahon, the former chief executive officer of Worldwide Wrestling Entertainment Inc. who ran unsuccessfully in 2010 against current Sen. Richard Blumenthal. McMahon has pumped $29 million of her own money into the race, which Murphy currently leads by 3 points, according to the RealClearPolitics poll average. That puts the GOP within the margin of error in a perennial blue state.