"I do feel he has demonstrated substantial good faith in that yes, he made a lot of money from bad places, but he's been very, very open about the fact that he's turned over a new leaf and is no longer taking money from those bad places and is instead spending to do good," Miller said.
The business records of wealthy candidates are often weaponized by rivals. Former President Obama cast GOP challenger Mitt Romney in 2012 as a ruthless plutocrat who made millions of dollars on corporate takeovers that put thousands of Americans out of work. Romney co-founded Bain Capital, a private equity firm.
Gray Davis won California's Democratic primary for governor in 1998 after portraying rival Al Checchi as a tycoon who pillaged Northwest Airlines, firing thousands and forcing thousands more to take pay cuts.
"When these wealthy, self-financing first-time candidates want to throw their hat in the ring, whether they're Democrat or Republican, they have to be prepared for a complete drill-down on how it is they made those millions of dollars," said Garry South, who was Davis' chief strategist.
As for Steyer, South said, "It's pretty hard for me to see a billionaire on the Democratic side credibly take on the whole issue of wealth inequality."
Within hours of Steyer's announcement, two of his opponents took shots at him.
"I'm a bit tired of seeing billionaires trying to buy political power," Sen. Bernie Sanders of Vermont told MSNBC.
Sen. Elizabeth Warren of Massachusetts, who is competing with Sanders for progressive voters, tweeted, "The Democratic primary should not be decided by billionaires, whether they're funding super PACs or funding themselves."
In an email seeking donations on Thursday, she said, "We need our candidates to compete to have the best ideas -- not just to write themselves the biggest checks."
Both Sanders and Warren, who frequently rail at what they see as unfair advantages for the super rich, have declined to take money from Wall Street donors.