With the early support of the state's Democratic leadership, Treasurer Bob Casey Jr. announced yesterday that he will seek the U.S. Senate seat held by Sen. Rick Santorum, R-Pa.

AP file photo Robert P. Casey Jr.

The move prefaces what is expected to be one of the most expensive and closely watched Senate races in the nation next year.

Shortly after Casey issued a widely anticipated statement declaring that he would pursue the 2006 race, Gov. Ed Rendell publicly endorsed his candidacy.

Rendell had offered a more significant gesture of support hours earlier as he persuaded former Treasurer Barbara Hafer, Casey's most prominent potential rival for the Democratic nomination, to drop her bid to challenge Santorum.

Casey, a former two-term auditor general and the son of the late Gov. Bob Casey, cited trade, health care and Social Security as central issues in his campaign.

Rebounding from a stinging loss to Rendell in the 2002 Democratic primary for governor, Casey won his current post last November with a record vote total for a statewide candidate.

National and state leaders of his party had prodded him to enter the Senate race in part because his relatively conservative views on social issues, including abortion and gun control, are seen as negating Santorum's ability to collect the votes of conservative Democrats.

While Santorum, the third-ranking Republican in the Senate, is viewed as a strong conservative, he has demonstrated his ability to appeal to Democrats in two House races in the Pittsburgh suburbs and in his two statewide victories.

Greeting yesterday's news with a statement in which he never mentioned his prospective opponent by name, Santorum said: "I look forward to a spirited debate in the issues of importance to Pennsylvania, such as saving and strengthening Social Security for future generations, and promising economic growth and job creation for hard-working Pennsylvanians."

Casey, too, placed special emphasis on Social Security in yesterday's announcement: "As senator, I will fight to protect and strengthen Social Security and will oppose any privatization plan that cuts benefits, requires massive borrowing from foreign countries or does nothing to improve the solvency of the Social Security system."

In an interview, Casey also criticized Santorum's support for Bush administration tax cuts.

"Certainly [I oppose cuts] for those making over $200,000 a year," Casey said. "When you make choices like that, they have real consequences. ... If you give trillions of dollars in tax cuts, you're not going to be able to address health care."

Casey declined to be drawn out on differences he might have with the incumbent on a variety of other issues, including foreign policy and the war in Iraq.

"As we go forward, we'll be talking about a whole series of issues," he said. "We'll have plenty to talk about down the road."

That road should be smoother for Casey because of Hafer's abrupt exit from the contest only two days after saying she would run and would not be deterred by her successor's entry into the contest.

"The governor has asked me to step aside and allow Treasurer Casey to run unopposed for [the] U.S. Senate," Hafer said in a statement. "After some consideration, I have decided to agree to the governor's request."

Hafer's words were well short of a ringing endorsement and she did not immediately respond to requests to elaborate on the statement.

Casey said, however, that he had spoken with Hafer after their complementary announcements.

"We had a very good conversation," he said, "very warm and positive."

Hafer's decision removed a significant hurdle for Casey, but he is still not guaranteed smooth sailing to his party's nomination.

Chuck Pennacchio, a Philadelphia college professor, said yesterday that he would seek the nomination. And with more than a year to go before the filing deadline in the race, there is plenty of time for other candidates to emerge, although Rendell and Rep. T.J. Rooney, the state party chairman, can be expected to try to quash any such effort. More liberal Democrats on social issues, including activists strongly in favor of abortion rights, pose the clearest potential source of intraparty opposition for Casey.

Casey expressed confidence that issues which he sees as most important in this race would trump Democratic differences.

"No matter where a voter stands on the issue of abortion, both pro-life and pro-choice are concerned about Social Security, about health care," he said.

Pointing to various initiatives of his years as auditor general, Casey said, "I've fought a lot of battles for children, for working mothers ... voters have had a chance to evaluate me."

Even at this early stage, a Feb. 16 poll conducted by Quinnipiac University suggested the potential for a close contest between Casey and Santorum.

The poll found that Casey led Santorum 46-41 percent. About 52 percent of the people polled said they approved of the job Santorum was doing in the Senate and 31 percent disapproved. The same survey showed Santorum in the lead against several other potential opponents, including Hafer.

Even before yesterday's developments, the state's 2006 Senate race figured to be among the most prominent and expensive contests in the coming election cycle.

Santorum is the most senior member of the GOP caucus facing re-election. He has the advantages -- and the baggage -- of incumbency and of his close ties to the Bush administration. Santorum has taken a leading role on issues from welfare reform to Social Security. He is known as a particularly prominent voice for, and beneficiary of, the rising strength of evangelical Christians within his party.

Casey's record-setting voter appeal rests in part on his close ties to traditional Democratic groups, notably organized labor. As auditor general, he compiled an aggressive record, presiding over investigations of issues such as nursing home standards and scrutinizing the investment performance of the state's retirement funds.