Bill Clinton’s poll ratings are in free-fall, and that surprise crash undermines the conventional wisdom that Hillary Clinton has a lock on the Democrat nomination.

A new CBS/New York Times poll shows that just 39 percent of American voters have a favorable opinion of Bill Clinton.

This is down from a 50 percent approval rating just a few months ago. In 2012, when Bill Clinton was campaigning aggressively for President Obama’s reelection, 66 percent of voters had a favorable opinion of Mr. Clinton.

Bill Clinton’s favorable rating today is actually lower than it was in 2008, when he last campaigned forcefully for Hillary as she was battling Barack Obama for the Democrat nomination. As that contest heated up, Mr. Clinton’s favorable rating sank to 46 percent.

A modest drop in Bill Clinton’s approval rating is to be expected as he reenters the political fray. As a former President, Clinton is normally viewed by voters as somewhat “above” politics, allowing them to hold more favorable views of the former politician.

Campaigning for one side in a political debate, even if that candidate is his wife, is naturally going to impact the opinions of those on the other side of that debate. The steep drop in Bill Clinton’s approval ratings, though, as he is only beginning to campaign for Hillary in the primary suggests something deeper is going on.

A few weeks ago, GOP frontrunner Donald Trump responded to criticism from Hillary Clinton by raising the issue of Bill Clinton’s sexual transgressions and the allegations of sexual assault against the former President.

In the wake of the controversy between Trump and Hillary Clinton, several women from Bill Clinton’s past emerged again from the media shadows to retell their stories of Mr. Clinton’s alleged sexual abuse.

The last time these allegations were raised at all in the media was back in 1998, during the height of the controversy surrounding Bill Clinton’s sexual relationship with White House intern Monica Lewinsky. Tellingly, during this time, Mr. Clinton’s approval rating also sunk to 39 percent in the CBS poll.

While the resurfacing of the old allegations brings back memories of a dysfunctional Clinton White House for older voters, for a large portion of the electorate, these stories are largely new. Voters younger than 35 weren’t even old enough to vote when the Lewinsky story dominated political news.

Interestingly, young voters are a powerful force behind the dramatic rise of Hillary Clinton’s current rival, Sen. Bernie Sanders. Sanders leads Hillary by 12 points among voters aged 18-34. In another poll, voters younger than 24 prefer Sanders over Hillary by a massive 42 points, 68-26.

It isn’t hard to imagine that these twin phenomenon — Bill Clinton’s plummeting approval ratings and Sanders’ surge among young voters — are related.

Before Trump, the conventional wisdom was that voters didn’t care about Clinton’s past sexual transgressions. These, the pundits assured us, were old news. For many voters, though, these allegations aren’t old news at all.

Even for those who do remember the old controversies, the kind of conduct allegedly committed by Bill Clinton is viewed much differently today than 20 years ago. This may be the clearest sign that the Clinton era is truly over.