Years of work and billions of dollars in political spending come to a head today in an election in which control of the House and Senate will be decided by a few dozen key races.

Democrats need to pick up six Senate seats to win control, but they have watched a number of previously vulnerable Republican incumbents battle back in recent weeks. The Republican senator of Montana, Conrad Burns, is now in a statistical dead heat with his Democratic opponent, Jon Tester, according to the latest Rasmussen poll.

It’s a similar story in Rhode Island, where Senator Chafee is now in a tie with the Democrat, Sheldon Whitehouse.

In Tennessee, the Republican, Robert Corker Jr., has retaken the lead against Rep. Harold Ford Jr. in the race to succeed Senator Frist. If elected, Mr. Ford would be the first African-American senator from the South since Reconstruction, a distinction that could also belong to Michael Steele, a Republican running for Senate in Maryland.

In Pennsylvania, Senator Santorum is trailing the Democrat, Robert Casey, by at least 10 points in recent polls, while Senator DeWine’s re-election hopes in Ohio are seen as fading. In Missouri, Senator Talent and his Democratic opponent, Claire McCaskill, have been running neck and neck for months.

Senator Menendez, a Democrat who was appointed this past January when Senator Corzine became governor of New Jersey, is facing a tough challenge from Thomas Kean Jr.

In Virginia, James Webb, a Democrat, has made his race against Senator Allen surprisingly competitive.

The battle for the House has Republicans on defense, as they try to protect Republican-held seats and stave off a Democratic wave. According to Congressional Quarterly’s latest election forecast, 33 Republican-held seats are either a toss-up, leaning Democrat, or Democrat-favored.

National polls show voters generically preferring a Democratic Congress, though Republicans have narrowed that gap in recent polls.

Rep. Rahm Emanuel, who’s leading the Democrats’ House campaign, says he is confident his party will net the 15 seats needed to control the House. In a conference call on Monday, Mr. Emanuel said, “It was inevitable that the Republicans on their side would start to kind of come back a little. But from early indications of our field operation, I feel strong about it.”

Republicans are trying to build on this momentum and party heavyweights have hit the campaign trail for House and Senate candidates. President Bush has crisscrossed the country in a 10-state push, telling voters that Democrats will raise taxes and Republicans will keep the country safe.

Well-known Democrats are also on the road and even getting out the vote via e-mail. President Clinton sent out his first pitch on Monday to members of MoveOn.org. “If we keep working until the polls close, we can take back Congress,” Mr. Clinton wrote. “Then we can roll up our sleeves and start building a better future together.”