In Pennsylvania this November, Democratic leaders see an opportunity not only to topple Rick Santorum, the Senate's No. 3 Republican, but also to begin writing their playbook for the 2008 presidential election.

Bob Casey Jr., the state's pro-life, anti-gun control Democratic treasurer, was tapped to enter the race last year by Sen. Charles Schumer, chairman of the Democratic Senatorial Campaign Committee, and Senate Minority Leader Harry Reid. Though members of the party's pro-choice base were initially unhappy, Casey's cushy lead over Santorum in most polls has emboldened pro-choice Democrats like Sen. Hillary Clinton to help him raise funds.

The youthful-looking, 46-year-old son of a popular former governor enjoys support across the state, a mishmash of urban blue-collar Democrats, socially moderate fiscal conservatives in the suburbs and religious conservatives in the exurbs. Casey received more than 3.3 million votes in his 2004 race for state treasurer — the most of any statewide candidate in Pennsylvania history.

Casey has held a double-digit lead over Santorum in most polls this year. Many Republican incumbents have seen their prospects dim along with President Bush's approval ratings, but Santorum's fate may be tied to Bush's unpopularity more than most.

"People who are dissatisfied with the president are overwhelmingly staying away from Santorum," said Chris Borick, director of the Muhlenberg College Institute of Public Opinion in Pennsylvania. "If there's a broader sense of misgivings about the direction of the nation and the people in charge, Santorum can't really escape that — he is a leader of the party and the third-ranking Republican in the U.S. Senate."

"It is very important for Santorum to show people that when he wakes up in the morning he's fighting for Pennsylvania, not Bush," acknowledged John Brabender, a media consultant and senior adviser to Santorum since 1990.

Santorum gained household-name status in 2003 when, in an interview, he appeared to equate homosexuality to bestiality. He accused the media of blowing his remarks out of proportion, but since then, he has been a favorite villain among liberals and some moderates.

With overwhelming support for Casey likely in the Democratic strongholds of Philadelphia and Pittsburgh and in the affluent suburbs of both cities, the national party can't be blamed for prematurely savoring a Santorum defeat. But some Democrats say they see trouble ahead in a candidate who is further right than most in the party.

"The bargain people thought they were making 14 months ago was a candidate who was personally opposed to choice but would not try to interfere with women's decisions. But now he's actually a candidate who's prepared to line up and behave like Rick Santorum," said Alan Sandals, a Philadelphia attorney who is running against Casey in the Democratic primary on Tuesday.

In a speech last year, Casey said he was in favor of overturning Roe v. Wade, the 1973 Supreme Court decision that legalized abortion throughout the United States. Casey also defied abortion rights supporters when he backed Justice Samuel Alito's elevation to the high court. Alito had served on the 3rd Circuit Court of Appeals based in Philadelphia before joining the Supreme Court in January.

By tapping Casey, the Democratic leadership is taking a gamble: Remove social issues from the table and Santorum's close affiliation with Bush and reputation for being intolerant could be the senator's undoing. But the strategy spurred a minor revolt in the party's base. Kate Michelman, former president of NARAL Pro-Choice America, bagged herself an endorsement from NOW after publicly considering a challenge to Casey.

While all nine Democratic women U.S. senators and other stars of the party have lent highly visible support to Casey's campaign, Sandals and fellow Casey rival Chuck Pennachio warn that an unexcited base could hand victory to Santorum.

"The message the national party and the state party have sent to Pennsylvania Democrats is: your vote doesn't count," said Pennachio, director of the history program at the University of the Arts in Philadelphia.

Pennachio said that voters he'd encountered on the campaign trail were angry that the party's presumptive candidate did not provide a starker contrast to Santorum. Indeed, liberal Web logs, no fan of Santorum, have been highly critical of Casey and the party's strategy in the state.

In fact, a Keystone poll released May 4 shows that Casey's lead over Santorum has slipped. Respondents favored Casey over Santorum 47 percent to 41 percent, compared with 50 percent to 39 percent in February.

G. Terry Madonna, director of Franklin & Marshall College's Center for Politics and Public Affairs and director of the poll, said while the results show Santorum isn't making substantial gains, Casey can't afford to put his campaign on autopilot.

"He has been on the receiving end of some third-party TV ads for six or seven weeks now on a couple issues," Madonna said. The ads portray Casey as friendly to lawyers and malpractice suits, and too busy campaigning to show up for work.

Casey has not come out strongly against those charges, a puzzling decision, Madonna said.

"He has not ratcheted up his campaign ... He is trying to get through this primary phase without exposing himself to his Democratic opponents, although they are making no headway," Madonna said.

Santorum aide Brabender said Casey can duck for now, but he won't be able to hide for long.

"Casey is being attacked constantly because he seems to be very shy about taking positions on the issues. He built up a big lead and didn't want to do anything to hurt it so he decided to run out the clock," Brabender said. "In the debates, he is not going to be able to hide from these things."

Casey declined to comment for this article.

Neither Pennachio nor Sandals has managed to crack double digits in polling or are expected to do so before Tuesday's primary.

Santorum is known as an aggressive and charismatic campaigner and debater who knows how to win the room. How Casey will fare when the campaign begins in earnest is difficult to predict.

"He is very mild-mannered, some call him dull and a little wooden," Madonna said.

"By almost any standard Santorum is seen as having more strength than Casey on those campaign characteristics," Borick agreed. "There is a concern that [Casey's] moderate tendencies might not energize enough people on the left and that they might not come out in November."

The Democratic leadership is betting that the visceral dislike many have toward Santorum plus unhappiness with Bush will be enough to entice voters to vote against the incumbent, even if it doesn't propel them for Casey. Casey also stands to scoop up the many swing voters who believe Santorum wrongly meddled in the right-to-die case of Terri Schiavo.

But political experts call that lackadaisical approach a bad strategy.

"Hatred doesn't let you win. You always have to give people a reason to vote for you," said Mary Anne Marsh, a Democratic strategist with the Dewey Square Group. "You can't say any Republican up for re-election is a proxy for Bush. Democrats have to make the case that they are better than the Republicans."

In any event, Democrats will be taking plenty of notes throughout the course of the campaign and in its wake.

A Casey victory might show that "you don't necessarily have to change the party platform, but having a few [pro-life] Democrats under your so-called big umbrella might be a good strategy to get majorities in the Senate and the House," Borick said.

But how to win fiscally conservative, socially moderate voters without alienating the base is a question that has haunted Democrats since 2000, when disaffected voters backed Green Party candidate Ralph Nader, which by several analyses cost Al Gore the presidency.

"A lot of Democrats are waking up to fact that the ground is much different now. A lot of pro-choice voters are not going to come over and vote for Casey. It could keep some of them at home, and the people who do all the hard work in campaigns tend to be pro-choice," Sandals said.