"Pennsylvanians deserve a voice in the U.S. Senate that will honor our values and fight for limited government, individual freedom and fiscal responsibility. I will be that voice," Toomey said.
Toomey's announcement coincided with "tax day" - the deadline for filing federal income tax returns - as fellow anti-tax activists planned protests nationwide.
Toomey, 47, had said this year that he was considering a bid for the governorship in 2010. But his sights shifted back to the Senate in March, after Specter bucked party leaders and cast one of three GOP votes - all in the Senate - to pass the $787 federal economic stimulus package that President Barack Obama signed in February.
Specter, 79, one of a dwindling number of moderates in an increasingly conservative GOP caucus, said he was voting his conscience, "not my own personal political interest." Toomey painted the stimulus package as part of a federal response to the recession that he said has put the nation "on a dangerously wrong path."
Toomey's announcement confirmed what had been virtually an open secret in recent weeks. At a gathering of Pennsylvania conservatives last month, Toomey received a standing ovation when he assured supporters that "it is very, very likely that very soon I will be a candidate for the United States Senate."
Specter was traveling in northeastern Pennsylvania and could not immediately be reached for comment, a campaign spokesman said.
Specter, who was elected to the Senate in 1980 and is the ranking Republican on the Judiciary Committee, won a fifth consecutive term five years ago - making him the first Pennsylvanian to achieve that distinction - after spending $21 million in the costliest Senate campaign in state history.
But he only narrowly overcame Toomey's surprisingly potent challenge in the primary - by a margin of barely 17,000 votes out of 1 million cast - even after then-Sen. Rick Santorum and then-President George W. Bush appealed to Republican conservatives to rally behind the incumbent.
In that campaign, Toomey sought to brand Specter as a RINO - Republican In Name Only. The incumbent dismissed Toomey as too "far out" for Pennsylvania and boasted about the hundreds of millions of federal dollars he steered to Pennsylvania annually because of his seniority. Specter outspent Toomey by about 3 to 1 in the primary campaign.
In recent years, Specter has battled Hodgkin's disease, a cancer of the lymphatic system, but maintains a busy schedule that includes daily games of squash.
Toomey said Wednesday he expected to benefit in the primary from Republican disenchantment with Specter's support for Democratic bills dealing with the economy. As for the general election, he claimed polling showed Specter in a weaker position than Santorum was before he was ousted by Democrat Bob Casey in 2006.
"Voters are just fed up with Sen. Specter," he said.
Sen. John Cornyn, the Texan who chairs the National Republican Senatorial Committee, said in an open letter to Pennsylvania Republicans late last month that Specter's re-election is "vital if we are to regain our majority in the Senate." The letter, prepared for Specter's campaign, cited examples of Specter taking positions that Democrats opposed.
"A vote for Arlen Specter is a vote for denying Harry Reid and the Democrats a filibuster-proof Senate," Cornyn wrote.
Toomey had headed the Club for Growth, a national conservative group that advocates smaller government and lower taxes, from the time he left Congress in 2005. The group was a major supporter of his 2004 campaign.
Prior to his election to the first of three terms in Congress in 1998, the Harvard-educated Toomey worked as an investment banker and operated several restaurants and bars in Pennsylvania with his brothers.
Specter, who had $7 million in his campaign war chest at the end of last year, put up a cable TV ad this month that sought to link Toomey's career as an investment banker to the current chaos in the nation's financial markets.
A likely wild card in the primary is the candidacy of Peg Luksik, an anti-abortion activist from Johnstown who ran for governor three times in the 1990s and who also is vying for conservatives' support.
Running as an independent in 1994, Luksik set a record for third-party candidates by attracting 13 percent of the statewide vote.
Toomey said he expected to raise "much more" money than the less than $5 million he raised in 2004 and vowed to wage a vigorous statewide campaign. A pilot, he said he will do some of his traveling in his own plane.
"It's a good way to get around a big state," he said.
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