"Sure, I'm going to support anyone against Pat Toomey," the likely Republican nominee, said Specter, who appeared second on the show. Losing "is not going to happen, but I will answer your question."
Sestak would not. "In a war, you always know you're going to succeed, and so I'm going to win, and I look forward to Sen. Specter's support after the 18th," he said. Pressed by Crowley, Sestak said, "Never deal with something that's not going to happen."
The Senate race - with its national implications, expense, and personal animosity - has drawn attention away from the more docile contest for governor, with six candidates in the two parties vying to succeed Gov. Rendell.
State Sen. Vincent Hughes endorsed Allegheny County Executive Dan Onorato in the Democratic primary for governor, making Hughes the first significant black politician in Philadelphia to break from the favorite son, State Sen. Anthony Williams. Onorato has been ahead in the polls, with Williams running second.
In a Muhlenberg College/Morning Call tracking poll released Sunday, Onorato led with 39 percent support, to 15 percent for Williams, 10 percent for Auditor General Jack Wagner, and 9 percent for Montgomery County Commissioner Joe Hoeffel. Twenty-six percent said they were undecided.
Sestak and Specter each drew 44 percent support, with 11 percent undecided.
The Muhlenberg results were based on interviews, conducted over four days, with 400 Democrats with a history of voting in primaries. The margin of error was plus or minus 5 percentage points.
The Senate candidates argued about who would be more electable against Toomey, who narrowly lost to Specter in the 2004 GOP primary.
In Media, in the heart of his House district, Sestak said that Specter would be "red meat" driving up GOP turnout in the fall, and that Democrats and independents wouldn't trust the party-switcher.
"We can't have a dead man walking," Sestak told reporters after the rally, according to the Associated Press.
Though Sestak may have been talking about electability, the remark could also be taken as a knock against Specter's age and past health problems. Specter, 80, is a two-time survivor of Hodgkin's disease; Sestak is 58.
"This is par for the course for Congressman Sestak," Specter's campaign manager, Christopher Nicholas, said Sunday night. "No wonder Gov. Rendell has described Sestak's campaign tactics as the 'worst of politics.' "
Specter, in an appearance at Doc's Union Pub in South Philadelphia, said he had beaten Toomey before and could do it again.
"You've seen my confidence, and you've seen my strong voice, and you've seen how labor is turning out on getting out the vote," Specter said. He added that Sestak "looks pretty nervous to me."
Democrats in Philadelphia were receiving recorded calls of President Obama urging a vote for Specter. Obama declined to visit the state in the closing days of the campaign, with White House aides eager to avoid damaging his prestige in a potentially losing cause.
In Johnstown, Pa., former President Bill Clinton campaigned for the Democratic nominee in the 12th District special election for the U.S. House, Mark Critz, who is running to succeed the late Democratic Rep. John Murtha.
Critz was locked in a tight race with Republican businessman Tim Burns, who has been riding a wave of discontent with congressional Democrats and the Obama administration. Republican Sen. Scott Brown, who won the special election to succeed the late Sen. Edward Kennedy (D., Mass.), stumped for Burns last week, and both parties are pouring money into the race.
Democrats have a registration advantage in the 12th District, but they tend to be more conservative than national Democrats, and the election is being closely watched as a harbinger of the fight for control of the House in the midterm elections. It is the only district in the nation that was carried by Democrat John Kerry in 2004 and by Republican John McCain in 2008.
Critz was a top district aide to Murtha, and Clinton called him the "how to" guy.
In the Republican gubernatorial primary, Attorney General Tom Corbett is heavily favored to defeat State Rep. Sam Rohrer, a favorite of conservative activists.
Gun control emerged as a last-minute issue in the Senate race, with Specter raising Sestak's F rating from the National Rifle Association in online ads in conservative areas of Western and northeastern Pennsylvania.
Sestak has sponsored legislation to reinstate the federal ban on the sale of assault weapons, which expired in 2004. Specter was still a Republican when he voted against a 10-year extension of the ban.
The assault-weapons ban has not been a dominant issue in a race that has turned on character, but Sestak's campaign clearly hoped that Specter's position would cost him support in Philadelphia, which polls show is his strongest region.
Meanwhile, state Democratic chairman T.J. Rooney called Sestak's unwillingness to promise support for Specter "incredibly troubling" and hypocritical.
"He's been calling Arlen Specter all sorts of names suggesting he's a better Democrat, but the one true-blue Democrat is Arlen Specter," Rooney said in an interview Sunday. "If Joe Sestak can't be counted on to support his opponent if he loses, what can we count on him for?"
Obama, Rendell, the state party apparatus, and most labor unions and county Democratic committees have backed the five-term Specter in his first race as a Democrat. Sestak is challenging him as the "true" Democrat.
A Republican for 28 years, Specter became a Democrat after his decisive vote for Obama's $787 billion economic stimulus plan, which crumbled Specter's support in the GOP. Sestak argues that Specter changed parties to save his skin, while Specter portrays the stimulus vote as a courageous act, and says the GOP has moved too far to the right for moderates like him.
Contested Pa. Primary Races
These offices will be up for election Tuesday. Also on the ballot will be races for the state Senate and House. Polls are open from 7 a.m. to 8 p.m.
K. Peg Luksik
Anthony Hardy Williams
H. Scott Conklin