Republican Sen. Rick Santorum, considered one of the most vulnerable incumbents, raised more than $3 million in the first three months of the year and has a 2-to-1 cash advantage over his leading Democratic opponent.

The Pennsylvania senator finished the quarter that ended March 31 with more than $9 million cash on hand compared to $4.5 million for state Treasurer Bob Casey, according to reports filed with the Federal Election Commission on Monday.

Santorum has raised more than $16 million for the November election. Casey has raised more than $8 million — $2.2 million in the first quarter of 2006, his campaign said.

The two are expected to face each other in what has been billed as one of the nation's most competitive Senate races. Polls have shown Casey with a double-digit lead over the two-term conservative lawmaker.