WASHINGTON - Is Joe Sestak the Democrats' only hope?

The former Delaware County congressman is running hard for a 2016 rematch against U.S. Sen. Pat Toomey (R., Pa.) in a race Democrats see as crucial to taking back the Senate.

So far, Sestak is the only potential challenger to show real interest - cheering those who praise his fight, but worrying some Democrats who chafe at the hard-charging ex-admiral, and who would prefer an alternative but have found none.

The race, theoretically, should have significant attractions. Democrats like their chances in a presidential election year, the national party will offer financial support, and the prize is a six-year term in the "world's most exclusive club."

But as the state's political insider class gathers in New York this weekend for the annual Pennsylvania Society booze- and schmooze-fest, the biggest news about the Democratic field is who isn't running.

"Most of the chatter by this point has been about candidates who have either taken themselves off the track or fallen down on the track," said Charlie Gerow, a veteran Pennsylvania Republican consultant.

An exception is Montgomery County Commissioners Chairman Josh Shapiro. He's the person most often mentioned as a potential alternative to Sestak. He's also one of the only options who hasn't ruled it out - though he also hasn't ruled it in.

Attorney General Kathleen M. Kane, once seen as a rising star and potential Toomey-slayer, has staggered through months of damaging missteps.

Treasurer Rob McCord took himself out of the running. Katie McGinty, who, like McCord, ran for governor this year but lost in the Democratic primary, seems to have foreclosed a Senate run by becoming chief of staff to Gov.-elect Tom Wolf.

U.S. Rep. Allyson Y. Schwartz, a Montgomery County Democrat leaving office after also running for governor, said this week, "It is not my plan to run for the U.S. Senate."

Shapiro, 40, is seen as a young, rising voice, but several political insiders said he may be eyeing the attorney general's seat. He declined to comment for this article.

"I don't think the bench, if you will, is particularly deep," said Chris Borick, a Muhlenberg College political scientist.

After a year in which four top-tier Democrats ran for governor, the dearth of Senate contenders has surprised many.

"Taking nothing away from Joe [Sestak], it's just hard for me to believe that he can get a free ride to a Toomey rematch," said Larry Ceisler, a Democratic analyst. "You'd think that there would be a slew of interested candidates."

If someone does hope to make a move, the Pennsylvania Society gathering could be a place to gauge support. Though the election is two years away, anyone hoping to defeat a well-funded incumbent needs to lay groundwork by recruiting staff and donors early.

"Now," said Alan Kessler, a Democratic fund-raiser.

"Last week," said Gerow.

"Months ago," said Ceisler.

Part of the reason is that the 2016 presidential race will soon create competition for staff and donations, Kessler said.

Another factor: the creep and cost of ever-longer campaigns.

Some ads in this year's Senate races aired a year and a half before Election Day. Candidates combined to spend $30 million or more in the top races, according to the nonpartisan Center for Responsive Politics - and those contests weren't in states with media markets as costly as Philadelphia's.

"To raise that level of resources, you can't get started late," said J.J. Balaban, who produced ads for Sestak in 2010.

Sestak hasn't waited.

"He's openly fund-raising and campaigning," said Joe Hoeffel, a former Democratic congressman from Montgomery County. "He hasn't officially declared, but he's certainly declaring privately that he's running."

Hoeffel once employed Shapiro as chief of staff, but is backing Sestak. "He deserves another shot at this and he's very well-qualified," Hoeffel said.

Another ex-congressman, Bucks County Democrat Patrick Murphy, also predicted Sestak would be his party's nominee.

Some Democrats, though, recoil at Sestak's driving personality and were upset that he ran a 2010 primary against Arlen Specter after the longtime Republican senator switched parties.

"Joe runs outside the lines, let's put it that way," Ceisler said. "Even though he does things in a very unconventional way, he does them very successfully."

Sestak is itching for another chance after beating Specter - and the party's establishment - and then coming within two percentage points of topping Toomey in 2010 despite a GOP wave.

An aide said Sestak had done more than 400 events to help Democrats in the last two years, met with volunteers in 37 of Pennsylvania's counties, and visited all 67 counties after the 2010 race.

His campaign had $1.3 million on hand as of his latest report, and he regularly e-mails supporters his critiques of Toomey.

One potential wild card: if a wealthy unknown jumps in, as Wolf did in this year's gubernatorial race. No such person has come forward so far.

Some worry Congress has become so toxic no one wants to join.

"You're talking about someone giving up their life, giving up their office or what they're doing now, to take on an incumbent, having to raise a ton of money, and ultimately going to Washington to accomplish what?" said Kessler, a Center City lawyer who has raised money for Toomey. "You just don't have a deep field as a result."

To Republican strategist Gerow, the thin field shows Toomey isn't as inviting a target as Democrats once thought.

He has a $5.4 million war chest (he'll be adding to it at a reception scheduled for Friday night at the Pennsylvania Society). And he has broadened his appeal by taking moderate stands on several high-profile social issues, Gerow said.

Democrats disagree. They cite Toomey's narrow 2010 win with a historic GOP wave at his back, and say the 2016 presidential race will bring out a Democratic-leaning electorate.

This week, the forecasting site Sabato's Crystal Ball rated Pennsylvania's Senate race a toss-up.

The analysis concluded by saying the Democratic establishment "appears to prefer a candidate named 'someone else,' " to Sestak, "but it's not clear that they have a better alternative."

jtamari@phillynews.com

@JonathanTamari