In an interview, Sestak called the Senate a broken institution but did not criticize Toomey by name. He said the Senate needed leadership.
"They are just moving from crisis to crisis and not confronting our problems," Sestak, 61, said. "We have a shorthand for it: debt ceiling, sequestration, super-committee. ... The biggest deficit we have is not the budget deficit, it's the trust deficit."
Pennsylvania Democrats are bullish about their chances of unseating Corbett, who has weak approval ratings in polls. There are at least seven declared or likely candidates for the party's nomination.
They include U.S. Rep. Allyson Y. Schwartz of Montgomery County, who has $3 million in her campaign account; State Treasurer Rob McCord, also of Montgomery County; former state Revenue Secretary Tom Wolf, a York businessman who has said he could spend up to $10 million of his own money on the race; and two former state environmental secretaries, John Hanger and Kathleen McGinty.
Questions about Sestak's plans intensified after he filed federal reports showing he had raised $460,000 in the first quarter of 2013 while he had yet to say what office he was seeking the money for.
Pennsylvania's Republican Party filed a complaint Monday with the Federal Election Commission contending Sestak was violating requirements that filers declare their intentions after raising at least $5,000.
Sestak, a retired admiral, was elected to Congress in 2006 to represent Pennsylvania's Seventh District, centered in Delaware County. He lost to Toomey in the Senate race by only two percentage points, despite running in a strong year nationally for Republicans and being outspent 2-1.
Earlier in 2010, Sestak defied party leadership - including the White House - and stayed in the primary race to defeat Sen. Arlen Specter, who had recently jumped from the Republicans to the Democrats.
In many ways, Sestak never stopped running after his 2010 loss. He was shaking hands at commuter rail stations at 6 a.m. the day after the election, then toured all 67 Pennsylvania counties to thank supporters. This spring, he has been visiting county party dinners across the state.
He also has been teaching at Carnegie Mellon and Cheyney Universities, and Dickinson College announced Tuesday that Sestak would hold the school's 2013-14 Gen. Omar Bradley chair in strategic leadership.
Approached just off the Senate floor Tuesday, Toomey declined to comment on Sestak's decision.
"Pat Toomey is focused on serving the people of Pennsylvania in the U.S. Senate and hasn't even thought about an election 31/2 years from now," spokesman Mark Harris said.
Long a leader on fiscal issues, Toomey probably strengthened his standing with suburban swing voters with his recent sponsorship of bipartisan legislation to expand background checks before gun sales. The bill fell short of the 60 votes needed to advance in the Senate.
Political analysts say Toomey, whose campaign fund has $2.4 million on hand, will be able to blunt a likely Democratic attack - that he's a right-wing zealot - by reminding voters that he defied GOP leaders and the party base on an issue as contentious as guns and worked across party lines.
A Quinnipiac University poll in April put Toomey's approval rating at 48 percent, compared to 30 percent who disapproved of his job performance. That was up from his 43-32 ratio in the poll in March.
Democrats running for governor put out statements Tuesday praising Sestak even as many in the party tried to sort out what the decision meant for the governor's race.
"There's a natural overlap between what would have been [Sestak's] base in a gubernatorial primary and Allyson Schwartz's base," said Philadelphia lawyer Mark Aronchick, who has raised money for Sestak and is lining up with Schwartz.
"It begins to make the field clearer," said Philadelphia-based Democratic strategist Daniel Fee. "But the race remains wide open."
Contact Thomas Fitzgerald at 215-854-2718 or email@example.com, or follow @tomfitzgerald on Twitter. Read his blog, The Big Tent, at www.philly.com/BigTent.
Inquirer Washington correspondent Jonathan Tamari contributed to this article.