Among the targets in his announcement video: fossil-fuel companies that he accused of torching the planet for short-term profits, drug companies he blamed for the opioid crisis and “banks screwing people on their mortgages.”

In the interview Tuesday, Mr. Steyer endorsed several policy stances that aligned him squarely, though not uniformly, with the progressive wing of his party. He said he supported decriminalizing unauthorized border crossings and expanding the size of the Supreme Court, and endorsed the creation of a government-backed health care option but not the elimination of private health insurance. People should shift toward government-backed care “by choice, not by fiat,” he said.

There is little doubt that a message of government reform has broad appeal to Democrats. But Mr. Steyer is also now on his third signature issue in little more than half a decade — after first championing climate change as a campaign topic, and then presidential impeachment — and he will have to compete with more than a handful of other Democrats trumpeting clean-government themes.

Some of those candidates can be expected to push back on Mr. Steyer’s self-presentation: Mr. Sanders, for one, said on MSNBC that he liked Mr. Steyer personally but was “a bit tired of seeing billionaires trying to buy political power.”

And Ms. Warren wrote on Twitter, in a clear reference to Mr. Steyer: “The Democratic primary should not be decided by billionaires, whether they’re funding Super PACs or funding themselves.”

Mr. Steyer also may have to defend his own business record, as the founder of Farallon Capital, an investment firm that had more than $20 billion under its management when he left in 2012.