Democratic White House candidate Bernie Sanders went on the offensive against front-runner Hillary Clinton on Sunday in the most contentious of their four presidential debates, accusing her of cozying up to Wall Street and misrepresenting his stance on health care and guns.

Reflecting Sanders' rise in opinion polls, the two battled with new urgency over who was best suited to lead Democrats in the November election. Sanders cast himself as the outsider who would lead a political revolution, while Clinton touted her experience and embraced President Barack Obama's legacy.

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In their last televised debate before Iowa's caucuses launch the nominating race on Feb. 1, Clinton raised questions about the self-styled democratic socialist's positions on Wall Street reform, health care and gun control.

Sanders pushed back at every turn. He painted Clinton as a defender of the status quo who accepted hundreds of thousands of dollars in speaking fees as a former secretary of state from Wall Street backers.

"I don't take money from big banks. I don’t get personal speaking fees from Goldman Sachs," the U.S. senator from Vermont said, adding, "I have huge doubts when people receive money from Wall Street."

He referred to his rising poll numbers in the early voting states of Iowa and New Hampshire, where he has pulled even or ahead of Clinton, saying he believed he could expand his number of supporters to include more African-American voters. He noted that when his presidential campaign began, Clinton was 50 percentage points ahead of him in the polls.

"Guess what: In Iowa, New Hampshire, the race is [now] very, very close," Sanders said.

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The debate followed a week of rising tension between the two leading candidates. Sanders was noticeably more animated than in previous debates, sometimes grimacing and shaking his head during Clinton's answers.

Former Maryland Governor Martin O'Malley, who lags badly in polls, was often a bystander although he joined Sanders in criticizing Clinton's Wall Street ties.

Clinton said she would build on Obama's agenda, accusing Sanders of voting to deregulate the financial market in 2000 in a way that led to the central causes of the financial collapse of 2008 that pitched the U.S. economy into a deep recession.

Clinton pounced on Sanders' "Medicare-for-all" plan that was announced just hours before the debate after Clinton had criticized Sanders for refusing to explain how he would pay for the proposal.

The former secretary of state, former U.S. senator and wife of former President Bill Clinton said Sanders' health care plan would undermine Obama's signature Affordable Care Act at a time when Republican legislators were still trying to repeal and replace it.

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"I have to say I’m not sure whether we’re talking about the plan you introduced tonight or the plan you introduced nine times over 20 years," she told Sanders. "But the fact is we have the Affordable Care Act. ... We have already seen 19 million Americans get insurance."

Sanders said he wanted to build on the Obama law by making health insurance more affordable.

Sanders supporters watching in Manchester, New Hampshire, said he seemed more engaged than in past debates. "I thought that Bernie was much more prepared," Chris Haigh, 66, said.

Sanders has pulled into a statistical tie with Clinton in recent polls in Iowa, whose caucuses are the first contest in the race to pick a nominee for the November election. He also leads Clinton in the next state to vote, Vermont neighbor New Hampshire, on Feb. 9, according to polls.