Bart Chilton, a commissioner on the U.S. Commodity Futures Trading Commission, announced Tuesday that he’s leaving his post. (Brendan Hoffman/Bloomberg)

The liberal firebrand of the Commodity Futures Trading Commission announced Tuesday that he plans to leave the agency soon, an unexpected move that further drains the commission’s leadership as its regulatory responsibilities swell.

Bart Chilton, who has served on the five-member commission since 2007, announced his departure at a meeting on a proposal to limit excessive speculative trading of commodities. He did not say when and why he’s leaving, or where he’s going. But he notified President Obama in a letter Tuesday of his plans to step down “in the not too distant future,” he said.

A Democrat who pushed to rein in Wall Street abuses, Chilton is perhaps best known for his mane of silver white hair and unconventional speeches, loaded with pop culture references and catchy phrases. “Cheetahs” was his name for the high-frequency traders who race “in and out” of the market.

When announcing his departure, Chilton again went the unconventional route, dropping the news on an unsuspecting audience just before the commission voted in favor of the so-called position limits proposal, a concept he has long supported.

“I’m reminded of this great Etta James song, ‘At Last,’” Chilton said.

“At last, we’ve got this rule here today,” and at last, he said, he can leave.

Chilton is bowing out at a time when the smallish CFTC is struggling to tackle big responsibilities mandated by the 2010 Dodd-Frank law. That law directed the agency to oversee a $400 trillion slice of the unregulated derivatives market, a major contributor to the financial crisis. The task thrust the little-known agency into the spotlight, along with its chairman, Gary Gensler, a former Goldman Sachs executive turned Wall Street reformer.

But as the CFTC forges forward, it may have to do so without a full commission.

The seat left open after Commissioner Jill Sommers resigned in July remains vacant. In August, President Obama nominated J. Christopher Giancarlo to succeed her, but the Senate has yet to confirm him.

Gensler has told the White House that he’s not interested in a second five-year term, and the president has not named a nominee to succeed him. Gensler’s term expired in April 2012. But he is allowed to remain in place until Congress leaves at the end of the year, or until his successor is confirmed and sworn in.

And now, Chilton will soon be gone. His term expired earlier this year, but he could stay through 2014, under the agency’s current rules. Chilton did not respond to requests for comment about his decision to leave. But past news reports have suggested that he was not going to be reappointed for another term.

If all three seats are left open, the CFTC will have only two commissioners in place: Republican Scott O’Malia and Democrat Mark Wetjen. The split along party lines could be a formula for gridlock when controversial policy issues arise, said Dennis ­Kelleher, chief executive of a nonprofit group called Better Markets.

“The CFTC could be shut down in many respects, and dysfunctional,” Kelleher said. “These two commissioners have very different views.”