July 7 (Bloomberg) -- The first television ad of Senator Rick Santorum's re-election campaign casts the Pennsylvania Republican as a staunch opponent of ``liberals like Ted Kennedy'' who favor helping undocumented immigrants attain legal status.

The ad doesn't mention that leaders of Santorum's party, including President George W. Bush and Senator John McCain, also favor that immigration approach. Nor does it mention that Santorum was one of only four Republicans to vote with Massachusetts Senator Kennedy and his fellow Democrats to raise the minimum wage.

Trailing by as many as 18 percentage points in polls, Santorum, the Senate's No. 3 Republican, is in try-anything mode. In addition to his immigration and minimum-wage stances, he has broken with Bush over treatment of terrorist suspects, while citing old, non-functioning chemical weapons shells buried in Iraq as justifying the U.S. invasion of that country -- a claim not even the administration has made.

Santorum's strategy is simple, said Terry Madonna, a political scientist at Franklin and Marshall College in Lancaster who runs the school's Keystone Poll: ``He's got to shake up the environment, or he's going to lose.''

Democratic strategists and independent political analysts say Santorum is one of the most vulnerable incumbents seeking re- election this year. A June 13-19 poll conducted by Quinnipiac University in Hamden, Connecticut, showed Pennsylvania Treasurer Bob Casey, the Democratic nominee, leading Santorum by a margin of 52 percent to 34 percent. Democrats need to pick up six Republican seats to gain control of the 100-member chamber.

A Governor's Son

Casey, 46, is the son of a former governor and, like Santorum, opposes abortion and tighter gun-control laws. The candidates' similarities have effectively neutralized those issues, leaving Santorum, 48, scrambling to find other themes that might help him regain lost ground before the November election.

Among other things, Santorum is seeking to play down his record of outspoken conservatism -- the American Conservative Union gave him a 96 percent rating two years ago -- while taking positions that may appeal to more centrist swing voters in Philadelphia's suburbs.

In addition to voting for an increase in the hourly minimum wage, to $6.25 from $5.15, Santorum last year broke with Bush by voting for a measure to bar the government from abusing detainees suspected of terrorism. He also bucked the president by supporting creation of a federal commission to ``critically'' examine progress in the war in Iraq and suggest improvements to the U.S. military.

Finding an Issue?

At the same time, by staking out a position on immigration to the right of Bush and his fellow Senate Republicans, Santorum ``thinks he's found an issue that could dominate the campaign,'' Madonna said. Santorum's TV ad says he is ``leading the fight to stop Bobby Casey'' and other ``liberals'' from ``giving amnesty and special tax breaks to those who entered our country illegally.''

Santorum visited the U.S.-Mexico border on May 30 and last month introduced a bill to add inspectors and border patrol agents. He says he is responding to a genuine public outcry over the Senate immigration measure, which differs from a version passed by House Republicans that stresses beefed-up border security.

``We have people crossing the border illegally,'' Santorum said at a July 5 town-hall meeting in the Pittsburgh suburb of Monroeville. ``It's a big issue in this country. It's a big issue in Pennsylvania,'' which he says loses almost $300 million a year in tax revenue from undocumented immigrants.

Courting the Base

Santorum acknowledged in the meeting that Monroeville has relatively few immigrants. Stressing the issue may help, though, with a particular group of activist Republicans the senator needs if he is to have a chance at victory.

``The Republican base is moved by immigration, and Santorum needs to get them out in full force,''' said Jennifer Duffy, who tracks Senate races for the nonpartisan Cook Political Report, a Washington newsletter.

Some of those voters may still be smarting over Santorum's role in the 2004 Republican Senate primary, when incumbent Arlen Specter, a supporter of abortion rights and the Senate immigration legislation, faced then-Representative Pat Toomey, who was ideologically closer to Santorum. Santorum joined Bush in campaigning for Specter, despite their differences; Specter narrowly won the race and went on to win re-election that November.

`They Really Hate Arlen'

``I can't tell you how deep it goes'' among Toomey backers, said Bill Green, a veteran Pennsylvania Republican operative who moderated Santorum's Monroeville meeting. ``They really hate Arlen, and they really thought Rick went out of his way to get Arlen re-elected.''

For his part, Specter, 76, said that Santorum's re-election is ``my No. 1 priority this year'' -- even though, as Senate Judiciary Committee chairman, he convened a hearing in Philadelphia this week to promote the Senate immigration legislation on the same day Santorum held his town-hall meeting. At Specter's side was Kennedy, the object of scorn in Santorum's TV ad.

Specter voiced disagreement with Santorum's characterization of the legislation, saying ``amnesty is a buzz word that has been used in derogation of what we are seeking to accomplish.''

Democrats say Santorum's focus on immigration is a sign of desperation. `He is trying to pander to a base that is angry about this issue,'' said Phil Singer, spokesman for the Democratic Senatorial Campaign Committee in Washington. ``When you are trying to appeal to your base, you are in a position of weakness.''

Hypocritical

Casey says Santorum is hypocritical on the border-security issue, citing seven instances in which the senator opposed increased spending to train more agents. In any event, campaign spokesman Larry Smar said, the challenger isn't hearing many complaints about immigration from voters. They voice concern about the loss of health insurance and other pocketbook issues, he said.

Despite the gap in the polls, Republicans say Santorum can't be counted out. The senator has a history of overcoming political odds: As a member of the House of Representatives in 1992, he won re-election with 61 percent of the vote after the state legislature redrew his suburban Pittsburgh district to bring in a heavily Democratic area. Santorum won the Senate seat two years later, edging Senator Harris Wofford by two percentage points.

``Is it a race? Yeah,'' said Green, the Republican operative. ``Is it a fight? Yeah. I don't think he's the underdog. I think he is where he always is.''

To contact the reporters on this story: James Rowley in Pittsburgh at jarowley@bloomberg.net; Laura Litvan in Washington at llitvan@bloomberg.net.

Last Updated: July 7, 2006 15:46 EDT