Sen. Bernie Sanders (I-Vt.) in September. (Matt Rourke/Associated Press)

During the Democratic primaries, Hillary Clinton’s supporters complained that Sen. Bernie Sanders had never even been a member of the Democratic Party. But since the Vermont senator conceded the race to his rival, he has been loyal to the Democratic nominee — endorsing her promptly, cementing her nomination in an important gesture of unity at the party convention and stumping for her on the airwaves and around the country. (Not to mention raising millions for down-ballot Democrats.) With Clinton’s lock on victory becoming clearer and clearer now, Sanders is looking ahead to after Election Day, telling The Post’s John Wagner that he has “leverage that I intend to use” to pressure Clinton from the left. Clinton supporters and the Democratic establishment might prefer he stay unwaveringly loyal, but using his newfound influence simply makes sense for Sanders — and actually could help Clinton in the long run.

Sanders’s surprisingly successful campaign shifted the policy debate in the United States. He proved what activists have been arguing for years: There is a strong constituency for progressive ideas such as a higher minimum wage, breaking up the big banks and an expansive effort to make college tuition free for millions of Americans. Thanks to Sanders’s efforts, they are part of the most progressive Democratic platform ever. But getting these policies into the party platform and Clinton’s stump speech is worthless without trying to see them become reality. And if these ideas make it into law, they’ll need strong executive branch appointees — at the Cabinet level and below — to implement them.

Defeat has not dented Sanders’s popularity; his favorability is not only higher than Clinton’s and Donald Trump’s — it is higher than President Obama’s. Fellow Sen. Elizabeth Warren (D-Mass.) — who like Sanders has been campaigning for Clinton wholeheartedly — has leveraged her support into real influence: As the New York Times reports, “policy negotiations [at the Obama White House] often include aides raising the caveat of ‘What would Elizabeth Warren say?'” There’s no reason that Sanders, working with Warren and other progressives, should not try to exert the same level of clout.

And yes, Clinton does bear watching. Virginia Gov. Terry McAuliffe (D) was likely wrong to say in July that Clinton would eventually support the Trans-Pacific Partnership. But Clinton’s friend and former co-chair of her 2008 campaign wasn’t making up her moderate views on trade out of thin air, which will affect what trade agreements under a Clinton administration would look like. Her transition team has been impressively leak-averse, but some of the names that have come out have raised eyebrows, such as Blanche Lincoln — a former senator, but more recently a lobbyist for companies such as Monsanto — as a “top contender” for agriculture secretary. Clinton’s record on financial deregulation and her hawkishness on foreign policy should also keep liberals hopeful but wary, and pushback from liberals will help curb any movement away from what Democratic voters want.

Besides, Clinton die-hards should want pressure from the left. Used correctly, it can only help in inevitable negotiations with recalcitrant Republicans. Pressure from Sanders and other progressives — Warren being the most prominent among them — can give Clinton cover to propose more liberal policies. Sanders’s (and Warren’s) popularity with voters can also help shift the public debate on Clinton’s proposals to the left. It will drive a Clinton White House crazy sometimes, but active pressure from Sanders is much more likely to help Clinton and the Democratic Party than to hurt them.