Washington — Sen. Bernie Sanders, the punch-from-the-gut self-proclaimed champion of the underpaid, overworked American worker, jumped into the presidential campaign Thursday as a very liberal alternative to Hillary Clinton.

The 73-year-old senator from Vermont enters the race as a decided long shot, but his candidacy suggests a sharp, perhaps divisive debate among Democrats from now until at least early next year over government’s role in American life and the influence of money in politics.

Sanders, the longest-serving independent in congressional history, has long been a strong, often lonely voice fighting against income inequality and for tougher corporate regulation. He caucuses with Democrats in the Senate.

He’s spent years building a loyal following of liberals who believe strongly that mainstream Democrats — notably the Clintons — are too cozy with Wall Street interests and too insensitive to the concerns of ordinary Americans.

Sanders avoided criticizing Hillary Clinton directly Thursday, saying he wants a “serious debate about serious issues.”

He was asked whether the Clinton Foundation’s acceptance of money from foreign interests was fair game for a political debate.

“I think what is more fair game … is the role of money in politics,” he said, and criticized the Koch brothers, big donors to conservative causes.

Sanders did offer some contrasts with Clinton. He recalled his 2002 opposition to the Iraq War, which Clinton at the time supported. She’s since said she got that vote wrong as a U.S. senator from New York.

Sanders also cited his ongoing battle against the proposed Trans-Pacific Partnership, which would make it easier for American corporations to do business in 11 other nations.

Liberals oppose the agreement, warning that it will encourage American corporations to rely more on inexpensive overseas labor. Aides to Clinton, whose husband pushed through the North American Free Trade Agreement in 1993, have indicated she could support the deal if it were shown to help American workers and national security.

The party’s liberal wing has been longing for a candidate such as Sanders, but he’s not their favorite. They still hope that Sen. Elizabeth Warren, D-Mass., will enter the race, though she has said she’s not interested.

Erica Sagrans, campaign manager of a group called Ready for Warren, praised Sanders, but added, “We need Sen. Elizabeth Warren in the race to make sure we have a Democratic nominee who will lead these fights all the way to the White House.”

Sanders does face daunting challenges, and not just from Clinton.

Former Maryland Gov. Martin O’Malley, praised by liberals for his state agenda, is expected to make a bid for the Democratic nomination, and former Rhode Island Gov. Lincoln Chafee is considering running. And should Clinton appear vulnerable, Vice President Joe Biden, among others, might jump in.

The allure for Sanders, as well as others, is that Clinton consistently gets only about 60 percent Democratic support in most polls. That suggests there’s a big chunk of voters who are willing to consider someone else.

Sanders’ chief asset is his tenacity. He’s been an underdog in nearly every race he’s run. He lost two U.S. Senate races and two governor’s races in the 1970s, breaking through with his barely successful campaign for mayor of Burlington, Vt., in 1981. He won by 10 votes.

Sanders called himself a democratic socialist, railing against what he termed unfair distribution of wealth and pushing an expansive government that would help create and find jobs.

The Brooklyn, N.Y., native was elected to Congress in 1990, then to the Senate in 2006, with aides telling anyone who asked that he wasn’t a socialist with a capital S, he was an independent.

He’s relentless in pushing his agenda. “How does it happen that the top 1 percent own about as much wealth as the bottom 90 percent?” he asked at a Capitol Hill news conference Thursday. “My conclusion is that type of economics is not only immoral, not only wrong, it’s unsustainable.”

Sanders protested a political system in which billionaires hold so much sway. “That’s a huge issue,” he said.

He offered contrasts between himself and Republicans on climate change and a desire to ease unemployment.

“Real unemployment in America is not 5 1/2 percent,” he said. “Real unemployment is 11 percent.” The best remedy, he said, is to rebuild America’s infrastructure.

Clinton has been trying to create a bond with the left. In her announcement video in mid-April, she declared, “The deck is still stacked in favor of those at the top.” On Wednesday, she called for an end to “the era of mass incarceration.” She’s met privately with Warren.

But she hasn’t detailed many specifics on how she’d ease income inequality.