Races to watch as the returns pour in : MarketWatch

Results from Indiana and Kentucky, which both contain key battlegrounds, will likely be the first to arrive, coming in after 7 p.m. Eastern.

Key tossups include Kentucky's Republican-held third and fourth districts. In the third, Rep. Anne Northup is attempting to fight off a tough challenge from Democrat John Yarmuth. Polls showed Northup holding a narrow lead in the Louisville-area district, which narrowly went for John Kerry in the 2004 presidential race.

A loss by Northup would signal that other battle-tested GOP incumbents in narrowly divided districts, such as Florida Rep. Clay Shaw, could be in for a rough night, wrote Nathan Gonzalez, an analyst at the Rothenberg Political Report.

Meanwhile, Republican Rep. Geoff Davis in Kentucky's more heavily GOP fourth district is facing a challenge from Democrat Ken Lucas. University of Virginia political science professor Larry Sabato rates the race as a toss-up. A loss by Davis would likely signal a strong tide for Democrats.

In Indiana, Republican Reps. John Hostettler, Chris Chocola and Mike Sodrel appeared on the defensive.

If Democrats are surging, it will be more apparent after 8 p.m. Eastern, as results start coming in from Ohio, Connecticut, Pennsylvania, Florida, Virginia, Missouri, New Jersey and Tennessee.

If Democrats are riding a major wave, Republicans could see their majority eliminated by the time the polls close in the 8 p.m. states, Gonzalez wrote. Republicans could consider losing only three or four seats as a "moral victory."

Three Connecticut Republican lawmakers -- Reps. Rob Simmons, Christopher Shays and Nancy Johnson -- remain engaged in fierce re-election battles. The Philadelphia suburbs have also turned into tough ground for GOP incumbents, and a handful of Republican-held seats in Ohio are viewed as in danger.

Democrats aren't invulnerable. Two Georgia Democrats -- Reps. John Barrow and Jim Marshall -- face stiff challenges. Sabato lists Barrow's race as a "toss-up," while Marshall is rated "leans Democrat." Other potentially vulnerable Democratic House members include Iowa's Leonard Boswell and Illinois' Melissa Bean.