WASHINGTON -- After four months of rigorous chemotherapy to treat Hodgkin's disease, Sen. Arlen Specter's physical transformation has been dramatic.

His wavy, reddish hair is gone, his once-full cheeks have hollowed, and his suits hang on his angular frame. Though he still plays squash many mornings with competitors half his age, he is visibly weaker.

But aides and colleagues say Specter has quickened the pace of his work, taking center stage on some of the most complex and politically explosive issues on Capitol Hill without much concern about whether he is aligned with his president or his party.

At 75, he may be in the twilight of his career, but he is also at the height of his power in the Senate, and he clearly does not plan to waste the moment.

With new power to control the agenda in his long-awaited role as Senate Judiciary Committee chairman, he recently held hearings in which he questioned the Bush administration's powers under the USA Patriot Act. This past week, he took on the legal rights of prisoners at the Guantanamo Bay Naval Base -- an issue the previous Republican chairman refused to bring up. Last month, he vowed to find a way to override a presidential veto on his legislation to expand federal funding for embryonic stem cell research.

In the all-consuming fight over Bush's judicial nominees, Specter dutifully ushered many of them through committee but even under heavy pressure, he refused to publicly state whether he would support Sen. Bill Frist's threat to abolish judicial filibusters.

"I have found that the more heavily engaged I am, the less time I have to think about how lousy I'm feeling," he said in an interview Friday in his sunlit office on the seventh floor the Senate's Hart building.

"But if I lie in bed in the morning, I don't feel like getting up, to be brutally frank about it. But I push myself out of bed and go to the squash court. ... And then I come in to the Senate and undertake a job which is really very demanding, very challenging and very invigorating."

Sen. Rick Santorum said he has, at times, been astonished by Specter's pace.

This week, the two senators held a field hearing together on youth violence in Philadelphia, after which Specter returned to Washington for meetings on his controversial bill to curb asbestos lawsuits. Specter describes the asbestos bill as "the biggest legislative challenge to come before Congress" in its history.

On Tuesday, the two Pennsylvania Republicans flew with President Bush to a $1.7 million fund-raiser for Santorum's re-election bid and for a speech to young future farmers in State College; Specter used the down time to engage Bush in a long conversation about stem cells.

After Wednesday's four-hour hearing on Guantanamo Bay detainees, in which he left the chairman's seat only to go to the Senate floor to vote, Specter called his Judiciary Committee aides to his house for one of his frequent working dinners. There, they fleshed out the specifics of legislation that will define who the Bush administration can classify as "enemy combatants" and what the rights of Guantanamo Bay detainees will be.

"The day is too short to get it all done," Specter said.

Some Specter observers have suggested that the combination of his ability to set the agenda on the Judiciary Committee and his diagnosis of Hodgkin's disease, a cancer of the lymph system, has given him a newfound sense of freedom to pursue controversial issues that directly conflict with the goals of the President.

That analysis ignores that Specter has always crafted his political persona around his independence. As far back as 1969, when he was up for re-election as Philadelphia district attorney, he and his running mate for controller chose the slogan: "They're younger, they're tougher and nobody owns them."

In the Senate, he has repeatedly infuriated Republican colleagues -- from his vote against Robert H. Bork, President Reagan's 1987 pick for the Supreme Court, to his attempt to use Scottish law to vote "not proven" in President Clinton's impeachment proceedings.

"Whether he has cancer or not, I don't think he would ever leave a fight unless he was carried out on a stretcher," said Specter's former Senate colleague John C. Danforth of Missouri.

When asked whether his illness has reshaped his priorities, Specter said it had sharpened his focus on his stem cell legislation -- which would grant federal funding to researchers who work with embryonic stem cells left over from in vitro fertilization treatments, as long as the cells are donated by the couples and would otherwise be discarded.

Specter says President Richard M. Nixon's "war on cancer" is an example of a lost opportunity that could have saved lives.

"If we spent as much money on the war on cancer as we've spent on other wars, I probably never would have gotten Hodgkin's," he said.

"How would you like to look in the mirror ... and find yourself bald and with splotches under your eyes and somebody would say to you -- 'It's just too bad your government didn't follow through on the war on cancer. Just too bad.' And there's a 110 million people out there."

Specter, who began his crusade on the stem cell issue in the late 1990s long before he got sick, appears to be increasingly comfortable using his experience as part of his arsenal. When Sen. Sam Brownback, R-Kan., recently challenged Specter on an ABC Sunday morning news show to define when life begins, Specter retorted: "Well, Sam, I'm a lot more concerned about this point about when my life is going to end."

Bigger battles ahead
Specter's greatest challenge this year as Senate Judiciary chairman could begin within the next two weeks when the Supreme Court is expected to wrap up its session and Chief Justice William H. Rehnquist, who is suffering from thyroid cancer, may announce his retirement.

But Specter said Friday that he had seen Rehnquist this past week at a lunch and wasn't so sure that a Supreme Court nomination would be next up on the Judiciary Committee's agenda.

"I have a speculation -- which I denominate as speculation -- that he's not going to retire," Specter said. "I say that because I have some special insights on what it's like to fight cancer and he's fighting cancer. I know what great therapy my job is for me. And I have an instinct that his job may be doing the same thing for him.

"I wouldn't go so far as to say it's keeping us all alive," he said with a wan smile. "But it's helpful.

"It's helpful to wake up in the morning and be chief justice and have a lot of important things to do. It's helpful to wake up in the morning and be chairman of the Judiciary Committee and think you have a lot of important things to do."