At first glance, it probably seems odd that so many Democratic Party presidential hopefuls are trashing the legacy of President Barack Obama.

The 44th president, after all, enjoys a 60 percent approval rating among Americans, according to Gallup — and that number spikes to over 90 percent among Democrats. One imagines someone with Obama’s popularity would be an invaluable ally in beating President Trump.

“The GOP didn’t attack [President Ronald] Reagan, they built him up for decades,” progressive think-tanker Neera Tanden warned Democrats like Cory Booker and ­Kamala Harris after the most recent presidential primary debate. “Dem candidates who ­attack Obama are wrong and terrible. Obama wasn’t perfect, but come on, people, next to Trump, he kind of is.”

But the Obama-Reagan comparison doesn’t quite work. Though Obama received hagiographic treatment from most of the media while he was in office, Reagan is, in every measurable way, a more enduring political figure in American life. Not only did Reagan lead a movement that conceived and enacted lasting legislative reforms, his presidency transformed the contours of our political discourse.

Add to that the fact that both ends of Reagan’s term are punctuated by achievements with historic resonance: first the lifting the 1970s economic malaise and then the imminent end of the Cold War.

Obama, on the other hand, presided over the slowest economic recovery in American history, and his most notable legislation, the Affordable Care Act, produced a divisiveness that paralyzed Washington for years.

While Reagan’s biggest successes were girded by a national consensus — he won 49 states in 1984 — Obama lost more than 1,000 seats on every level of government while leading his party to minimal policy gains. Most of Obama’s “accomplishments” were the result of executive fiat — pen and phone, as he put it — and thus easily undone.

One of the most successful promises of the 2016 Trump campaign, in fact, was to roll back Obama’s unilateral actions, many of which used executive power to compel Americans to adopt progressive cultural attitudes and live with laws Democrats couldn’t actually pass.

All of which is to say: Democrats massively overestimate the power and meaning of Obama’s legacy.

It’s almost certain that Joe Biden’s early popularity in the primaries is predicated on his perceived moderation and ability to win in the general — and considering the gaggle of ­socialists running against him, this isn’t exactly a surprise.

“I hope the next debate we can talk about how we fix the things that Trump has broken, not how Barack Obama made all these mistakes,” Biden groused last week.

Obviously, many of the ­attacks on Obama can be chalked up to simple conventional politics. Biden, the front-runner for the 2020 nomination, is invested in praising the successes of his former administration.

And, as others rightly see it, one of the best ways to dislodge Biden from this perch is to be critical of that inheritance. And let’s just say the Obama record is a target-rich environment for progressives.

If Biden is going to portray Trump administration deportations of illegal immigrants as a second Kristallnacht, for instance, he should be able to explain why it was OK for the Obama ­administration to deport more illegal immigrants than any White House in history.

But this fissure within the Democratic Party is directly tied to Obama’s legacy. The former president inflamed progressives, empowered and normalized them, but never quite delivered on his lofty promises of change. Even as Obama set the stage for his party’s hard-left shift, he was forced — to his endless frustration — to live within the realities of a representative republic.

Even recently, Obama — just like the progressives running against his legacy — praised Medicare for All as a “good new idea,” despite the fact that its success would mean the repeal of his signature achievement, ObamaCare.

If Obama doesn’t believe in his legacy, why should his ideological progeny cling to it?

In reality, the former president has always been something of an anomaly: a politician who, though personally admired by many voters, is a terrible advocate for anything other than himself. Some Democrats, it seems, have tricked themselves into believing that Obama’s policies, rather than the man, are popular.

Because, in the end, Obama’s most notable policy accomplishment might have been ushering in the presidency of Donald Trump.

Twitter: @DavidHarsanyi