The competitive race in Oklahoma for the U.S. Senate has escalated into a slugfest in recent weeks, with three high-profile Republicans fighting for the heart of the party, as the likely Democratic candidate, Rep. Brad Carson, quietly waits for an opponent.

The GOP primary -- a tight battle between former representative Tom Coburn and former Oklahoma City mayor Kirk Humphreys -- could end up in a runoff, political experts say.

This week, Humphreys -- declaring "the gloves are off" -- unleashed ads attacking Coburn for his votes in Congress against spending increases for intelligence and airport security.

Coburn has refused to respond to Humphreys's criticism. Meanwhile, the major third candidate, state corporation commissioner Bob Anthony, has accused Humphreys of "shady" business and land deals that Anthony said raised ethical issues because Humphreys was a public official. Humphreys has denied wrongdoing.

Meanwhile, Coburn, a practicing physician, caused a political uproar this week by declaring in an interview that he favors the death penalty for "abortionists and other people who take life." Coburn was unavailable for comment, but a former aide said that Coburn was simply raising an issue and had no intention of suggesting legislation.

Carson declined to comment. But Humphreys -- who opposes abortion -- called the comment "over-the-top rhetoric."

"I think Tom is a purist, not a realist, and I think in Washington you need a realist to get things done by seeking to move other people," said Humphreys, who has the support of the state GOP establishment in the July 27 primary. "I don't see him doing that. He's someone who introduces 125 amendments to the farm bill. He left Washington a very frustrated fellow, and I suspect if he goes back, he'll still be frustrated."

Coburn left the House in 2000 to fulfill a commitment to term limits. The maverick conservative tried unsuccessfully to overthrow then-House Speaker Newt Gingrich (R-Ga.), and was skewered by many for asserting that the critically acclaimed Holocaust film "Schindler's List" was offensive to show on television. Polls show him running slightly ahead of Humphreys.

"He's not in the mainstream. He's on the fringe," said one prominent Oklahoma Democrat who asked that his name not be used. "And that's the campaign against him."

Sources close to Carson say the moderate congressman would probably prefer to run against Coburn because it would be easier to portray him as a extremist, one who fights only to preserve his ideological beliefs -- and not for the state's interests.

Humphreys counts as his political base Oklahoma City, the state capital and largest city, while Coburn has a strong following in Tulsa, adjacent to his former district. Both candidates have been aggressively advertising in the Tulsa market.

Carson is strongly favored to win over three lesser-known candidates in the Democratic primary. He has raised $3 million and expects to raise another $3 million in a race that will attract national attention in the fight for control of the Senate.