President Bush is expected to be in Pittsburgh later this month to help U.S. Sen. Rick Santorum raise money to defend his Senate seat against an anticipated challenge from Democratic state Treasurer Bob Casey Jr.

A White House spokesman said he could not provide scheduling details for the president beyond his current trip to India, but organizers of the Pittsburgh event said it would take place in a private home in the closing days of March.

It would be the president's second fund-raising visit for Mr. Santorum, R-Pa., in this election cycle. Last June, Mr. Bush appeared at a Santorum event in Bryn Mawr, Montgomery County, raising a reported $1.7 million for the senator and the state Republican Party.

National Republican leaders, including presidential adviser Karl Rove and Ken Mehlman, the Republican National Committee chairman, have described Mr. Santorum's re-election as at the top of the White House's political priorities for the 2006 election cycle.

The president is a double-edged ally for Republican candidates. His public approval ratings have languished in recent months, and Democrats have tried to capitalize on that relative loss of popularity by tying his record to GOP candidates such as Mr. Santorum.

Mr. Bush, however, remains a fund-raising powerhouse, and Mr. Santorum is intent on amassing the resources he will need in what is expected to be one of the most expensive Senate races in the nation this year.

By the end of 2005, Mr. Santorum, with $14 million and $7.7 million in cash on hand was near the top of the list of incumbent Senate fund-raisers. Mr. Casey, with nearly $6 million raised and $3.3 million in cash was at the top of the list of Senate challengers.

Mr. Santorum has consistently trailed his anticipated challenger in public opinion polls in the closely watched race, but Republican partisans argue that those numbers are an unreliable guide to a contest that won't be resolved until November.

Before then, both candidates have to secure their party's nominations and both are heavy favorites to do so. Mr. Casey faces Chuck Pennacchio, a Philadelphia history professor, and Alan Sandals, a lawyer, in the Democratic primary. John Featherman, a Philadelphia real estate broker, plans to challenge Mr. Santorum for the GOP nomination.