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.

"The map has shifted in ways I don't think anyone expected," said Nathan Gonzales, deputy editor of the nonpartisan Rothenberg Political Report.

Of course, Republicans aren't alone in expanding the traditional electoral map. Democratic Senate candidates are running competitively in normally safe Republican territory, such as North Dakota, where former Attorney General Heidi Heitekamp, a Democrat, is in a dead heat with Republican Rep. Rick Berg. In 2008, Republican presidential candidate Sen. John McCain handily won the state by more than 8 points.

"Half of the Republican-held seats (up for election) are labeled 'toss ups'," said Matt Canter, spokesman for Senate Democrats' campaign arm. "It's an incredible development."

Elsewhere, costly gaffes have transformed races. In this cycle's marquee example, Missouri Sen. Claire McCaskill, previously considered the Senate's most vulnerable Democrat, now leads Republican Rep. Todd Akin by an average of 5 points in the wake of his controversial comments about "legitimate rape" earlier this year.

Rounding out the list of other toss-up states: Arizona, Indiana, Massachusetts, Montana, Nevada and Pennsylvania.

"Republicans would need to win eight of the nine most competitive races on our list in order to take the majority," Gonzales said, stressing that the path would be much easier if the GOP can hang on to seats in Nevada, Massachusetts, Indiana and Arizona. "So that's possible -- it's just not as likely."

Brian Walsh, spokesman for the Senate Republican campaign arm, acknowledged a competitive landscape but said the GOP is confident about its chances.

"There's less than three weeks to go and there's roughly a dozen Senate races that are within the margin of error so clearly it's a very competitive battle, but Republicans are well-positioned to pick up seats on November 6," Walsh said.

Still, political analysts differ on the margins but seem to agree that if the election were held today, Democrats would most likely retain control of the chamber. The Rothenberg Political Report estimates anywhere between no net change and a Republican gain of four seats, which would flip the chamber. By contrast, University of Virginia Prof. Larry J. Sabato's Crystal Ball tentatively pegs the outcome at 52 seats for Democrats and 48 for Republicans - a scenario in which Republicans only pick up one net seat.

By all accounts, fewer surprises are expected in the House, where Republicans maintain a 240-190 advantage, excluding five current vacancies. All lawmakers in the chamber face an election, but analysts say it's unlikely Democrats can take back control given the fact they need a total of 218 seats to do so.

The Rothenberg Political Report expects anywhere from four to 10 Democratic pick-ups, while Sabato predicts a net gain of seven for Democrats.

Depending on how well Romney does in Florida and Ohio, there is a chance he could help make congressional races in those swing states competitive, Gonzales said, adding that Republican candidates are not running strong enough to win in the event of a Romney loss.

Whether it's President Obama or Romney sitting in the Oval Office come January, odds are that Americans should be prepared for more divided government and -- if past is prologue -- more legislative gridlock.