May 5, 2009 -- Sen. Arlen Specter's decision to become a Democrat was supposed to be the easy path to reelection.
So much for that.
Specter's move ignited a firestorm in Pennsylvania and beyond. It's left him with new enemies in both parties, even though he has a set of powerful friends that includes President Obama and Vice President Joe Biden.
Rep. Joe Sestak, D-Pa., told ABC News that while he hasn't made up his mind, he's closer to seeking the Democratic Senate nomination than he was before the party switch, given the way the Democratic establishment has sought to rally behind Specter before Pennsylvania voters get a choice.
"How this was done gives me grave concerns. That's not the ideal that we came to Washington for," Sestak said.
"I really was taken [aback] by the Democratic political Washington establishment saying, here's your candidate, Pennsylvania," he added. "I thought, when I came here, and when President Obama came here, we weren't for an establishment -- we're for change."
Specter, 79, now faces the real possibility of both strong primary and general-election battles.
While Sestak and others weigh primary candidacies, former governor Tom Ridge, R-Pa., is being courted by state and national Republicans to carry the GOP banner. Already, former Rep. Pat Toomey, R-Pa., -- who almost beat Specter in the 2004 Republican primary -- is in the race, and raising money at an impressive clip.
Sestak said he will be closely monitoring Specter's statements and votes over the coming months, to make sure he's helping the country move forward on areas including healthcare expansions and national defense.
"Arlen has a lot of questions to answer. If they aren't answered appropriately, I do believe we should have someone else carrying the banner for the Democratic Party, and for all Pennsylvanians," he said.
White House Press Secretary Robert Gibbs on Tuesday reiterated the president's support for Specter, even if Sestak or others challenge him in a primary.
"I think the president was pretty clear on this. Senator Specter has his full support and he'll do what -- what's necessary to see him re-elected," Gibbs said.
Sestak or another primary challenger can expect the help of liberal activists, many of whom have been waiting years or decades to defeat Specter.
In addition, rifts are appearing inside the Democratic Party that could cost Specter the support of organized labor, despite the Democratic establishment's promise to rally behind its newest member.
Top officials of the two largest national unions told ABC News Monday that they are unlikely to support Specter as long as he opposes their top legislative priority, the pro-unionizing bill called the Employee Free Choice Act.
"If a candidate isn't good for workers, we won't be there. If they are good for workers, we will be there regardless of their party," Richard Trumka, the secretary-treasurer of the AFL-CIO, said on ABCNews.com's "Top Line." "I mean, we supported Arlen Specter -- and he was a Republican -- because he was good for what was happening. He was good for our members at that time."
Andy Stern, president of the Service Employees International Union, met with Sestak Monday in Washington and conveyed to him the importance his members are placing on the Employee Free Choice Act.
"There's no way they're ever going to be supporting someone who is seen as thwarting this opportunity," Stern told ABC after the meeting. "It is hard to imagine any union supporting a candidate in the Democratic Party for the U.S. Senate who doesn't have strong positions on both healthcare and Employee Free Choice."
Specter supported the measure -- which opponents call "card-check" and denounce as a job-killer -- until this year, when he announced that he would oppose it.
Even if he gets through the Democratic Primary, he may wind up facing a Republican all-star in the general election. Ridge, a former governor who remains popular in the Keystone State, is considering a Senate bid.
The former Homeland Security secretary would have to face Toomey, a former House member, in a tough primary. Still, the more moderate Ridge -- like Specter, he supports abortion rights -- is facing national and state-level pressure to take on a senator now viewed as a turncoat in his old party.
A Quinnipiac University poll released the week showed Specter only narrowly leading Ridge in a hypothetical match-up, 46-43.
"There's a lot of pressure on him right now. He's being beseeched by Republicans -- and not just moderates," said Terry Madonna, a professor of public affairs at Franklin & Marshall College in Pennsylvania. "There is growing alarm in the Republican Party that if Toomey's the candidate, he will lose."
Toomey's campaign announced Tuesday that he'd brought in more than $500,000 in less than three weeks in the race. Toomey or whoever secures the GOP nomination can expect a surge of national dollars; already, the Republican National Committee and other party groups have launched fundraising pitches based on Specter's flip.
The more immediate concern for Specter remains the Democratic primary. The state and national party establishment -- including President Obama, Vice President Joe Biden, Gov. Ed Rendell, D-Pa., and Sen. Robert P Casey Jr., D-Pa. -- have already lined up firmly behind Specter, but that doesn't mean there won't be other challengers.
Sestak, 57, a second-term House member and former Navy admiral, may not be swayed by the array of powerful Democrats urging him not to run, said Larry Ceisler, a veteran Pennsylvania Democratic consultant.
"He's not beholden to the Democratic establishment, and he's not beholden to President Obama," Ceisler said. "I never thought they could clear the field for Arlen Specter."
That's Sestak's attitude as well.
"I honestly believe that leadership matters," he told ABC. "He admits that he failed to shape the Republican Party. But then he left it. We don't do that in the military."
Some are skeptical of organized labor's threats regarding Specter. Labor unions have generally supported Specter in the past, and leaders may not want to pick a fight with Democrats including Obama, Biden, Rendell, and Casey, Madonna said.
Indeed, the SEIU's Stern said that what will matter is Specter's vote on whatever the Employee Free Choice Act looks like in its final form.
"No one's going to get exactly what they want," he said. "So the question is, where is his flexibility, on not letting his idea of perfection stand in the way of progress."
Looming over all of the positioning is Specter himself, a crafty politician who has learned to win under changing political circumstances and demographics.
"Running against Specter," said Ceisler, "is like getting your teeth pulled without the Novocain."
ABC News' Jake Tapper contributed to this report.