He remained nearly invisible until last week when, seemingly out of nowhere, Thompson burst back into the 2013 mayoral field as a top contender after raking in $1 million in the latest filing period.
“It is completely different this week,” said one high-ranking City Council member, echoing powerbrokers across the city who sounded blindsided by the change. "The million-dollar filing changed the game."
The influx of cash, which was more than any other mayoral candidate hauled in during the last six months, coincided with a surge of speaking engagements. He gave his first major policy address at a breakfast thrown by the Association for a Better New York, and at a debate in Harlem, Thompson drew enthusiastic cheers as he vowed to oust NYPD Police Commissioner Raymond Kelly as the city's top cop.
At a public health forum in Downtown Brooklyn, he railed against the current administration for using “one shots and fiscal gimmicks” to balance its budgets, and slammed Bloomberg’s large soda ban as nothing more than “big PR.”
"Anything before this week was spring training,” said Eduardo Castell, a Thompson campaign adviser who was also the manager of his 2009 bid. "This week, the umpire yelled, ‘Play Ball!’ And Team Thompson is ready to play."
Thompson’s staffers said the reappearing act has been in the works for months. The former comptroller recently scaled back his day job at a municipal finance firm to part-time so he could spend more time speaking at churches, meeting with community leaders and addressing local Democratic clubs.
“He has been keeping a remarkably busy, pedal-to-the-metal schedule for the last couple of months,” Castell said.
He also ramped up his fundraising, making an aggressive push in recent months that resulted in more than $1 million. They made their strongest push for high-income donors to boost their haul as quickly as possible.
Thompson stunned observers in 2009 when he finished within five points of Mayor Michael Bloomberg in a David vs. Goliath race that saw him outspent by tens of millions of dollars.
This time, he's facing a crowded field of Democratic challengers, and hoping to paint himself as the level-headed grown-up, with public and private sector experience and a play-it-safe agenda that stresses education, public safety and economic growth.
But there are challenges.
Thompson has never been a rock star. Throughout the 2009 campaign, a less-than-friendly press frequently questioned his ability to connect with voters.
“In 2009, he was a lackluster candidate,” said Baruch College professor Doug Muzzio, who maintained the closeness of the race had more to do with a backlash against Bloomberg and term limits than voters' connection with Thompson.
To win now, Muzzio said, Thompson will have to be a very "different candidate than he was in 2009."
In a wide-ranging interview last spring as he geared up for the campaign, Thompson rejected that assessment.
“You don’t get that percentage just by being there," he insisted. "You never get that percentage in people just saying, ‘I don’t like the other guy so I’m voting for you.'"
Thompson also stressed things will be different this time. Instead of running against one of the richest men in the country, Thompson is facing City Council Speaker Christine Quinn, Public Advocate Bill de Blasio, and City Comptroller John Liu.
“I don’t think that there is any sense of inevitability for anyone. There is no incumbent. There is no person that’s going to spend $120 million. I think it’s a very level-playing field this time around,” Thompson said.
That means the winner will depend on who connects with voters, he said.
“It really is going to be a question of: Who appeals to the voters? Who speaks to them about the future and a vision that takes in all New Yorkers?” he added.
And Thompson has some clear advantages. For one, he's the only black candidate in the race. In 2009, he carried an overwhelming 75 percent of black voters and 55 percent of Hispanic voters. He won Brooklyn and The Bronx, too.
That support was on full display in Harlem this week, when the Bed-Stuy-born Thompson drew enthusiastic cheers at a Martin Luther King Jr. day forum, where Quinn was booed.
He was especially powerful when he recounted having to teach his 15-year-old stepson how to act during a stop-and-frisk.
"He looked at me and said, ‘But Bill, if I’m not doing anything wrong, why would I be stopped?" Thompson recalled. "And I had to say to him, ‘Well, because you’re black.'"
Conventional wisdom, Muzzio has said, is that, as the only black candidate in the race, Thompson has an automatic base of 25 to 30 percent, which at least guarantees that he makes it into a runoff.
But this round of contenders may disrupt that equation. Liu, for instance, also enjoys support in the black community. And de Blasio, whose wife is black, has traditionally had strong outer-borough and minority appeal, political observers say.
Opponents note that Thompson has less than $2 million in the bank, which means he will have to post stellar numbers through the rest of the campaign if he wants to reach the spending caps that Quinn has already hit.
There's also that pesky question of whether Thompson has the fire to win.
At the ABNY breakfast last week, one audience member raised the issue of Thompson’s even-keeled temperament, which some have read as disengaged or even boring. But Thompson said people were misreading his consensus-driven approach.
“If you don’t scream and holler, if you don’t attack people in New York City, you’re calm and mild-mannered,” he said. “I think that my style is the style of bringing people together, collaboration, but then making decisions and moving forward is one that best-serves New York City."
But Muzzio said Thompson will have to prove his passion over the coming months.
“The bottom line is," he said, "you’ve got to walk the walk.”