By Terence Samuel -- U.S. News and World Report

MEADVILLE, PA.--The Assembly Hall at the Meadville Medical Center is a basement-level affair with blue and brown carpet and folding chairs and tables. The atmosphere this rainy Friday afternoon is damp and a little dark, but it's not just the weather. It is also partly the tough questions for U.S. Sen. Arlen Specter.

Specter, a moderate Republican, is seeking his fifth term in the Senate, and this is a chance for him to talk to doctors about medical liability insurance. "I practice very defensive medicine," says one doctor, describing his fear of being sued and the spirals in his malpractice insurance. "I do a lot of CAT scans; I put a lot of people in the hospital."

The White House and the GOP leadership in Congress want to cap liability damage awards at $ 250,000, but Specter--as he often does--is bucking the party line. He generally supports the caps but believes allowances must be made for the most egregious cases. "It is very uncertain how all of this is going to work out," he says.

Specter may as well have been describing his own re-election prospects. Specter's almost 24 years in the Senate is as long as any Pennsylvania senator's in history, but at 74, he finds himself in the political dogfight of his life, just for the nomination of his own party. It is a battle with implications beyond the simple calculus of his own race; indeed, the Keystone State primary has become a proxy fight over whether there is still a role for moderates in the increasingly conservative GOP.

The contentious April 27 primary is playing out in a crucial presidential battleground. The contest pits Specter, a sometimes contrarian, pro-choice Republican from Philadelphia, against Pat Toomey, a doggedly conservative three-term congressman from Allentown, who accuses Specter of not really being a Republican. The winner will face Democratic Rep. Joseph Hoeffel in November.

A tough fight. Specter has always had a difficult relationship with his party's conservatives. He does not share President Bush's zeal for endless tax cuts; he is pro-choice, and he opposed the Supreme Court nomination of Robert Bork, a watershed moment that infuriated the right. The well-funded Toomey presents Specter with his first real primary test; Specter has won every previous primary with more than 65 percent of the vote. "I don't think a lot of the frustration with Specter has ever been channeled," Toomey says. And Specter doesn't seem to disagree. "I've got a tough primary fight," he acknowledges to a gathering of Crawford County Republicans. "I make no bones about it."

The outcome may ultimately hinge on a larger question: Is there room in the Republican Party for members who less enthusiastically subscribe to the reigning conservative agenda on God, guns, and the primacy of tax cuts? "There is going to have to be room because if there isn't room, they won't have a party," says Specter. "When I came to the Senate, there were a lot more of us," he adds, invoking defeated or departed moderate Republican senators like Connecticut's Lowell Weicker, Missouri's John Danforth, and Oregon's Mark Hatfield.

But Toomey, 42, a smooth-talking, Harvard-educated former investment banker, argues that Republicans are well situated to advance a more conservative national agenda. With a conservative Republican president and GOP control of both houses of Congress, he says, it would be prime time for conservatives but for the efforts of "a certain number of Republican senators who didn't buy into the idea of the Republican Party in the first place." He adds: "We need Republicans who believe fundamentally in cutting taxes, controlling spending."

Some of Toomey's attacks seem to be resonating. "Every five years [Specter] comes back to the right," says Tim Platt, a self-described gun activist at a Specter event here. But Specter's biggest worry has been the infusion of almost $ 2 million from the Club for Growth, a Washington antitax group that has made the defeat of Republican moderates its goal. "They want my scalp so that they could make other [moderate] senators behave," says Specter. Specter, asserts Club for Growth Executive Director David Keating, "has had one of the worst records of any Republican on fiscal restraint."

On the other side of the intraparty fight is the Republican Main Street Partnership, an organization of moderate Republicans that exists, according to its executive director, Sarah Chamberlain Resnick, to "protect our members against the Club for Growth."

Says Chamberlain Resnick: "I think there are a lot of Republicans who are upset with this infighting, and frankly it's costing a lot of money, money that I would prefer to be spending against Democrats."

Interestingly, it's the government spending decried by Toomey and the Club for Growth that could be Specter's salvation. The senator is widely perceived as a heavy hitter who has delivered for the home folks. "People are beginning to understand what seniority means," brags Specter.

Seeing pork. Of course, others view it somewhat differently. Last year, the budget watchdog group Citizens Against Government Waste named Specter the most prodigious purveyor of pork in Congress. In the 2004 budget, CAGW says, Specter amassed $ 3.5 million in pet projects, including $ 1 million for Allentown to develop computer mapping and $ 500,000 for the Pittsburgh Police Department for a video-surveillance system.

Toomey, who rails against such expenditures, has modeled himself after the state's junior senator, Rick Santorum, a hard-charging, antiabortion crusader who has become one of the champions of the party's conservative wing. But Santorum, the chairman of the Senate Republican conference, has endorsed Specter. "I just think he's our best chance to hold on to the seat," says Santorum. "If we want to be a majority here in Washington, we are going to have to be a big tent." Toomey thinks the moderates ought to come from somewhere else. "There is room in the party for anyone who wants to be in the party," he says, "but the people of Pennsylvania deserve to be represented by someone who shares their values."

Polls show Specter up by as much as 15 points. And the old warhorse has something else going for him as well. "Twenty-four years of seniority," says Santorum, "means something to the people of Pennsylvania." Just how much it means will be clear in a couple of weeks.