By Janet Hook -- Los Angeles Times

Sen. Arlen Specter of Pennsylvania, an embattled Republican with strong backing from President Bush, narrowly defeated a conservative challenger Tuesday in a primary widely viewed as a proxy fight over the GOP's ideological direction.

With 98% of the state's precincts reporting, Specter had 51% of the vote to Rep. Patrick J. Toomey's 49%.

Tuesday's tight race vote culminated a long, expensive and bitter fight between Specter and Toomey, a little-known House member who accused the incumbent of straying too far from their party's conservative principles on tax cuts, abortion rights and other issues.

The primary contest was watched nationally because of its potential impact on this year's presidential race and the Republican battle to keep or expand its narrow Senate majority.

Pennsylvania is among the largest states viewed as up for grabs between Bush and the presumed Democratic presidential nominee, Sen. John F. Kerry of Massachusetts.

Specter argued -- and many analysts agree -- that it would be harder for Bush to carry the narrowly divided state if the more conservative Toomey were on the ticket with him.

In remarks to supporters in Philadelphia before the final result was known, Specter acknowledged the tough fight he had faced.

"It's been a very, very difficult campaign," he said.

In the general election, Specter will face Democrat Joseph M. Hoeffel, a House member from the Philadelphia suburbs who easily won his party's Senate primary Tuesday.

A key question raised by the GOP primary is whether Specter can win over the large number of Republicans who voted against him.

Hoeffel said Tuesday night that Specter's "vulnerability was clearly demonstrated in this primary.''

A Senate loss in Pennsylvania could jeopardize Republican control of the 100-member chamber, where the party has 51 seats.

And Bush would have a much harder time getting his agenda through Congress if he won a second term but his party lost its Senate majority.

The Pennsylvania GOP primary was viewed as a potential bellwether for the party as a whole.

The contest turned into a high-profile test of strength between the party's staunchest conservatives and its dwindling moderate wing -- an ideological division that Bush had tried to straddle by defining himself as a "compassionate conservative."

Steve Moore, president of the Club for Growth -- a conservative group that poured money into Toomey's campaign -- said he was disappointed but not surprised that Specter won because of his enormous financial advantage and the weight of Bush's endorsement.

"This was always a David vs. Goliath situation," Moore said. "Goliaths usually win."

Moderate Republicans hailed the result as a victory for those who believe the GOP needs to have broad appeal to centrists as well as conservatives if it is to enjoy broad electoral success.

"There are some places in this country that moderate Republicans need to hold -- or else the seats will become Democratic,'' said Sarah Chamberlain Resnick, executive director of the Republican Main Street Partnership.

Toomey, 42, ran a strong race against Specter by tapping into conservative frustration not just with the 24-year Senate veteran but with the Republican national leadership. Toomey, from Allentown, and other conservatives have fumed as Bush and congressional GOP leaders have supported big increases in spending, including last year's expansion of Medicare.

Specter, 74, fought back vigorously with arguments that Pennsylvania could not afford to lose his seniority and clout in the Senate, where he has been able to funnel billions of dollars for projects and aid to the state.

Specter, a former lawyer in Philadelphia, is among the best-known of the dwindling ranks of Rockefeller Republicans-- moderate GOP politicians from the Northeast. The drain on this wing of the party accelerated in recent weeks, as two New York Republican congressmen -- Amo Houghton and Jack Quinn -- announced they would not run for reelection.

The GOP has become increasingly dominated by conservatives as its base shifted to the South and West in recent decades.

Many of these conservatives refer to the likes of Specter as RINOs -- Republicans in Name Only.

Despite Toomey's loss, conservative activists hope his showing will send a cautionary message to other moderate Republicans: They, too, could face career-threatening and expensive primary opposition if they stray from their party line.

For most of the campaign, Specter held a commanding lead in the polls over Toomey.

But his margin narrowed sharply dramatically in recent weeks. Specter apparently reaped big political benefits from a campaign appearance for him last week by Bush.

The president also was featured in one of Specter's last television ads. "I'm going to say it as plainly as I can: Arlen Specter is the right man for the United States Senate," Bush said in the ad. "I'm proud to say, I think he's earned another term."

Hoeffel will try to use Bush's role to his advantage.

Specter, he said, "needed Bush to drag him over the finish line. He's demonstrated he's [Bush's] senator. He's not Pennsylvania's senator.''

Others in the GOP establishment rallied to help Specter. Virginia Sen. George Allen, chairman of the National Republican Senatorial Committee, urged Specter's colleagues to chip in if they had not already.

At least 17 Republican senators donated a total of $102,500 to Specter, according to an Allen aide.

Against that formidable array of establishment power, Toomey's surge in the polls was all the more noteworthy. In the final weeks of the campaign, it became clear he had tapped into the ire of conservatives over Specter's defections from the party line. He blocked President Reagan's nomination of Robert H. Bork to the Supreme Court, refused to vote for convicting President Clinton in his impeachment trial and joined with other centrists in efforts to scale back Bush's 2001 tax cut bill.