Bernie Sanders was on a roll. "I think what Secretary Clinton just said is absolutely right," the Vermont senator said at the eighth Democratic presidential debate. "I think I said it many months before she said it, but thanks for copying a very good idea."

In many ways, Sanders has been a perfect primary opponent for Hillary Clinton. He is just competitive enough that he keeps her organization nimble and fresh for the general election, but unless Clinton ignores the challenge, he has no path to beating her for the nomination.

The Michigan primary was a case in point. Sanders won, defying all the polls and showing that his populist economic message could have legs with union voters even in a racially diverse state. Clinton still won more delegates that night, thanks in part to her Southern firewall, further padding a lead Sanders is unlikely to ever overcome.

But Sanders is having one effect on Clinton that could prove a liability in the general election. As he noted in the debate, she is increasingly adopting his positions and moving to the left politically.

It's hard to imagine Clinton opposing the Trans-Pacific Partnership deal, negotiated by a Democratic administration, without Sanders' challenge. She briefly gestured in the direction of a public option, pronounced dead during the Obamacare debate but revived by Sanders' Medicare-for-all push in the primaries. She belatedly came out against the Keystone XL pipeline and vowed to "defend and expand" Social Security after facing what she described as Sanders' "false innuendos" that she would make cuts.

When Sanders introduced a bill in September to ban private contractors from providing correctional services, Clinton endorsed the move on social media within a month.

While her husband, former President Bill Clinton, remains popular with Democrats and liberals, progressives have been quietly rebuking his business-friendly, more centrist Democratic Leadership Council politics for nearly two decades.

Ralph Nader won nearly 2.9 million votes running to the left of Al Gore in 2000 as the Green Party nominee, arguably costing the Democrats Florida and the election. Howard Dean brought this argument into the party's primaries in 2004, campaigning as the candidate of the "Democratic wing of the Democratic Party."

Barack Obama effectively won the argument in the 2008 primaries. He said he wanted to be a transformative president like Ronald Reagan, except in the opposite ideological direction, not like Bill Clinton. He defeated Hillary Clinton for the nomination by marrying the Dean vote to a supermajority of African-American Democrats. Once in office he reportedly dismissed a suggestion by then-Chief of Staff Rahm Emanuel that he scale back his liberal ambitions on healthcare reform with a derisive shot at the Clintons: "I wasn't sent here to do school uniforms."

Sanders has pushed this thinly veiled debate about the Clinton legacy further. He cheerfully embraces the socialist label, leading to a revival among Democrats willing to do the same. A Des Moines Register poll, for example, found over 40 percent of Iowa Democrats willing to call themselves socialists.

Clinton has effectively repudiated some of her husband's anti-crime policies that critics say contributed to mass incarceration, especially among African-Americans; welfare reform; capital-gains tax cuts and Wall Street regulations. This has some Clinton foes licking their chops.

"Throughout the course of this campaign, Secretary Clinton has repeatedly been forced to the left by her avowed-socialist primary opponent, Bernie Sanders," Jeff Bechdel, communications director for the anti-Hillary America Rising political action committee, told the Washington Examiner. "Desperate to avoid a repeat of 2008, Clinton has shown a startling willingness to say or do anything to win, and that includes flip-flopping on decades-old positions just to score political points. It's no wonder a majority of Americans think she is dishonest and not trustworthy."

At the same time, many of the old centrist Democratic positions were adopted to deal with the party's political liabilities during the Reagan era. Clinton is betting times have changed and that the votes for Sanders are proof.