Pat Toomey and Specter have been campaigning against each other for weeks, although Toomey hadn't made a formal announcement and the primary is a year away, The Philadelphia Inquirer reported. Toomey came close to beating Specter in 2004, losing the Republican primary by 17,000 votes out of 1 million cast.
Toomey has staked his ground with opposition to U.S. President Barack Obama's stimulus plan and other economic policies. He said he decided to enter the race after Specter voted for the stimulus package in February, infuriating many Republicans.
"We can stop the bailouts and the spending stampede; we can reduce the burdens on taxpayers; and we can unlock the ingenuity and job creation potential of our great nation once again," Toomey said on his Web site.
"I am running for the U.S. Senate because I believe the economic stakes for our country have never been higher."
Specter, who turned 79 in February, is in his fifth term in the Senate.