President Bush may be fighting nationally to keep his own job, but here in Oklahoma, his coattails are giving quite a lift to the Republican candidate in a tight and contentious Senate race.

For weeks, former congressman Tom Coburn (R ) has been struggling just to stay even with Rep. Brad Carson (D), a troubling sign in a conservative state where Coburn should be enjoying a comfortable lead by now. But as the race comes to a close, the family doctor who prides himself in snubbing the Washington political game has found that his ace in the hole is the president.

Carson and Coburn are locked in one of the nastiest and closest Senate races, hurling insults at each other via millions of dollars' worth of TV ads. As one of a handful of races that will determine whether the GOP retains control of the Senate, the contest is attracting vast resources from the national parties and outside groups.

The campaign has become so personal that both sides are worried the nonstop attacks may be alienating voters. Political analysts were stunned this week when statewide polls showed the percentage of undecided voters increasing, an indication they say that people are not happy with the choices.

"I haven't seen two candidates tear each other like this in years. It's particularly ugly," said Keith Gaddie, a political science professor at the University of Oklahoma.

But Pat McFerron, a GOP pollster who has been tracking the race, said the undecided voters are largely committed Bush supporters. While they may have reservations about Coburn, Republicans say, they could swing Coburn's way out of loyalty to the president, who is running 30 points ahead of the Democratic presidential nominee, Sen. John F. Kerry (Mass.), here.

Coburn obliterated the opposition in the summer primary and began the general election in a strong position. But Carson launched an aggressive and frequently negative media and ground campaign that for a while kept Coburn off balance. The two-term congressman and Rhodes scholar has presented himself as a moderate good old boy, featuring ads with him in his pickup truck. Although his is taking help from the Democratic Senatorial Campaign Committee, Carson has distanced himself from Kerry -- and no national Democrat has been allowed to set foot in the state on his behalf.

Carson's strategy from the start has been to portray Coburn as something of an extremist and a gadfly. He reminded voters that Coburn refused to support some pork-barrel projects for Oklahoma when he was a member of Congress, and he made sure reporters were informed whenever Coburn made an inflammatory comment.

Coburn has long had a penchant for impolitic remarks. During the campaign, he said he favored the death penalty for abortionists, he called the Senate race a choice between "good and evil," and he said he had heard there was "rampant lesbianism" in Oklahoma schools. And in a state with a large Native American population, Coburn disparaged age-old federal treaties that fund the tribes and criticized some Cherokees with marginal bloodlines for claiming tribal benefits. By late September, Carson -- who is part Cherokee -- was inching ahead. Earlier that month, Salon.com published an article alleging that Coburn had committed Medicaid fraud and sterilized an underage woman without her consent. Coburn denied the charges and accused the Carson campaign of planting the story -- an accusation that Carson denied.

The initial coverage was damaging to Coburn. But after Carson aired a negative ad about the charges, Coburn fought back, and his campaign began to rebound. "I think the Carson campaign pushed it one step too far and Carson's negatives went up," Gaddie said. "Coburn caught some sympathy."

Republicans also began taking advantage of Bush's popularity in the state. The National Republican Senatorial Committee (NRSC) has been airing widely an ad declaring that a vote for Carson is "a vote against President Bush " -- a direct plea to Bush supporters who are worried that Bush's second-term agenda could be hampered by a Democratic Senate. Another independent group has purchased radio ads calling Coburn a "fearless conservative" who "stands with President Bush."

To reinforce the point, Vice President Cheney, Majority Leader Bill Frist (R-Tenn.) and former president George H.W. Bush have all been here campaigning, urging voters to support Coburn for the sake of the Senate and the president.

Coburn and the NRSC have also hurt Carson by tarring him as a liberal and comparing him to Sen. Edward M. Kennedy (D-Mass.) and Sen. Hillary Rodham Clinton (D-N.Y.). "That alone can have a devastating effect in this race," said Kenneth S. Hicks, a political scientist at Rogers State University in Oklahoma.

Today, most polls show Coburn slightly ahead but within the margin of sampling error. Democrats and Republicans concur that the contest could pivot on how well Bush does. "We have made it clear from the get-go that in places like Oklahoma -- where the president won by 20 points in 2000 -- that we would take advantage of that lead," said Patrick Davis, NRSC political director. "The voters don't want to be responsible for the Democrats taking over the Senate." State Democratic Party Chairman Jay Parmley said, "There is no question that the wider the president's lead, the harder it is for us."

Rep. Brad Carson (D) gained ground with attacks, up to a point.Republican Tom Coburn alienated some with his impolitic remarks.