NEW YORK — Eliot Spitzer's fledgling bid to return to politics neared a key deadline Thursday, when thousands of petition signatures were due only four days after the tarnished ex-governor launched his New York City comptroller campaign.

He had until midnight to collect at least 3,750 valid petition signatures to get on the Democratic primary ballot for September.

"We're doing well. For tonight, we'll be comfortable with what we file with," the Democrat told Fox 5 News while out for a run early Thursday. He wouldn't specify how many signatures his campaign had gathered.

Experts say it's difficult, but doable, to garner that many signatures in so few days. Spitzer himself noted Wednesday night that "the number is big."

Spitzer, who resigned amid a prostitution scandal in 2008, startled the political establishment and shook up what had been a tame comptroller's race by jumping into it, a decision he says he just made over the weekend.

City candidates were allowed to start petitioning in early June, and many did. Campaigns generally gather two to three times as many signatures as needed, in case some are challenged as having incomplete addresses, missing dates or other flaws.

Spitzer has said he's aiming for 7,500 signatures. His self-financed campaign has offered canvassers $12 an hour to collect them.

Some candidates submit far more signatures than needed in an effort to broadcast widespread support. Spitzer's Democratic rival, Manhattan Borough President Scott Stringer, amassed more than 100,000 signatures through an all-volunteer effort, his campaign said.

"One hundred thousand people signed my petition, and we didn't have to pay anybody," Stringer said Thursday evening on TV news station NY 1's "Road to City Hall."

Republican candidate John Burnett said Thursday he'd filed about 8,000 signatures. Petitions weren't yet due for some third-party contenders, but Libertarian Kristin Davis' campaign has collected 3,100 so far, spokesman Andrew Miller said. Adding an unusual element to the campaign, Davis is a former madam.

Green Party candidate and former teacher Julia Willebrand's campaign didn't immediately respond to a message sent through the campaign's Facebook page Thursday evening.

The signature process can be painstaking. Spitzer and Stringer each need signers who are registered Democrats, live in the city and haven't signed another comptroller hopeful's petition. Signers must supply their names and addresses and date the forms, and signature-gatherers also have to fill out certain information.

Spitzer's four-day timeframe is certainly a challenge, said Jerry Skurnik, a longtime New York Democratic consultant who isn't working with Spitzer.

"It could be done, if they spend enough money and they're really organized," Skurnik said.

Meanwhile, Spitzer got encouraging news Wednesday from a poll showing he topped Stringer 42 percent to 33 percent among registered Democrats, including those leaning toward but not settled on a candidate. The Wall Street Journal-NBC 4 New York-Marist questioned 546 registered Democrats and had a margin of error of plus or minus 4.2 percentage points for questions limited to them.

Spitzer said Wednesday night that while the poll numbers were "comforting," he was aware he had more work to do.

"I'm never confident," he said. "And that is defensive politics."

Stringer campaign manager Sascha Owen expressed confidence that as more voters get to know Stringer, "he will be the obvious choice for comptroller."