WASHINGTON — On Tuesday afternoon, when most senators were preparing to leave Washington for the holiday recess, Tom Coburn was declaring his intention to stick around.

"The floor's going to be open," said the 59-year-old Oklahoma Republican. "I'm going to have to be here ... to try to stop stuff."

Stopping stuff is Sen. Coburn's specialty. In a Congress that has had trouble passing even the simplest legislation, Sen. Coburn, who proudly wears the nickname "Dr. No," is a one-man gridlock machine. This year, the senator, who indeed is a medical doctor, single-handedly blocked or slowed more than 90 bills, driving lawmakers on both sides of the aisle to distraction.

He blocked a ban on genetic discrimination by health insurers. He thwarted a bill to set up a program to track patients with Lou Gehrig's disease. Also nixed: an effort to promote safe Internet use by children and a resolution to honor the late environmentalist Rachel Carson on the 100th anniversary of her birth.

A bill that would authorize government mapping of the ocean floor and coastal areas? No way. One that would require more data collection on the availability and quality of broadband service? Uh-uh. If Dr. Coburn had his way, there would be no new funding for a Justice Department office to investigate unsolved Civil Rights-era killings, no promotion of wild-land firefighter safety.

Sometimes, Dr. Coburn, an obstetrician who sees patients one morning a week, disagrees with the proposals. As a fiscal conservative, he usually objects to what he sees as excess spending. Sometimes, he just wants to force a debate or improve on items that would otherwise fly through the Senate. In a crowded legislative calendar, not everything gets the scrutiny voters might imagine.

Dr. Coburn's weapon of choice is the "hold," a procedural maneuver that allows a single senator to prevent a bill from being passed quickly without a roll-call vote or floor debate. Until a rule change this year, senators could keep their holds secret, and they usually did. Dr. Coburn notifies colleagues about his intentions.

To keep track, Dr. Coburn has four manila cards in the pocket of his suit coat. He pulls out the list, printed in tiny type on both sides, whenever colleagues approach to discuss their bills. In his office's intranet, which staffers jokingly call the "Write-Wing Portal," there's a section for aides to look at bills that have incurred their boss's displeasure.

John Kerry, the Massachusetts Democrat and 2004 presidential nominee, had several bills blocked by Sen. Coburn this year. One would help military veterans and reservists with small businesses. Dr. Coburn said it duplicated another program. Another would expand government-backed venture-capital investment in small business. "I can't imagine the government can be good at it," Dr. Coburn said.

Negotiations between the two men stretched into Tuesday night as the Senate tried to finish tax legislation and a mammoth spending bill. Dr. Coburn's remaining issues were "this point, that point, the other point," complained Sen. Kerry. "You can't have a one-person committee for every single bill that goes through the U. S. Congress. You'd never get anything done."

Dr. Coburn first ran for a seat in the House in 1994, prompted by his opposition to the Clinton health-care plan. He was part of a wave of Republican freshmen who promised to shake up the system and cut government waste. He became an irritant to Democrats and Republicans alike. In 1999, he delayed passage of a Republican-backed appropriations bill by proposing 115 amendments.

In 2000, Dr. Coburn kept a campaign promise to leave after three terms. He returned to Muskogee to be with his wife, Carolyn — who was Miss Oklahoma of 1967 — and to treat patients. But Dr. Coburn made another run in 2004, entering a Senate race relatively late.

Sent back to Washington, he started pushing pet topics, one of which is to warn of the dangers of sex. In the past, he has held a slide show for young staffers, depicting the effects of sexually transmitted diseases and the consequences of sex outside marriage.

In February, after the Democrats took over, Dr. Coburn wrote to his colleagues, warning of his intention to block fast-track passage unless his "common-sense principles" were met, which included: New programs can't duplicate existing ones, and new spending should be offset with other cuts.

"I think it's good for our leadership to know there's a very vocal and effective right flank in our party, and that can be used in negotiations with Democrats," says Republican Sen. Jim DeMint of South Carolina, a conservative who often works closely with Dr. Coburn.

The alliance with Mr. DeMint didn't keep Dr. Coburn from slapping a hold on a bill Mr. DeMint supports — the one that would set up a government registry to track people with Lou Gehrig's disease, also known as amyotrophic lateral sclerosis. Mr. DeMint is one of 69 senators sponsoring the bill. "He's a physician and he's going to have his opinion on that," says Sen. DeMint, who says he thinks he can win over his colleague.

This month, Dr. Coburn sent out a letter encouraging senators to talk to him about legislation they wanted to complete. That had some Democrats muttering about a "tyranny of one." Last week , Senate Democrats took to the floor complaining about Republican tactics, which Senate Majority Leader Harry Reid called "obstruction on steroids." They pointed to the lack of action in several areas stymied by Dr. Coburn, including the Lou Gehrig bill.

"Just give us a chance to bring that up on the Senate floor," pleaded Sen. Dick Durbin of Illinois, who is the majority whip. "How much time would it take? Thirty minutes?"

Mr. Reid, calling Dr. Coburn's tactics "unreasonable," said when the Senate returns next year, he plans to combine a number of blocked bills and bring the package to a vote.

Dr. Coburn says he isn't to blame for slow progress. Democrats used too much floor time debating the Iraq war, he says, and tried to move too many bills on the fast track. He notes he didn't try to stop the spending bill, the only legislation Congress technically must pass in order to keep the government running.

As Congress wound down, Dr. No started to act a bit like Dr. Maybe. After winning some changes, he agreed to let one of the Kerry bills pass. Dr. Coburn dropped objections to giving the Federal Housing Administration leeway to insure more troubled mortgages, a key part of the administration's response to the housing crisis.

"I lost," Dr. Coburn says. "I decided because of the severity of the problem we face, I can't win that point...but I can at least debate it."

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