Presidential contender Liz Warren’s strategy is to win the Democratic nomination by promising the most free stuff. On Monday, she upped the ante: She is offering a whopping $50,000 in student loan forgiveness for nearly everyone who borrowed for college or graduate school. That’s old-fashioned, Tammany Hall-style bribery — handing out dollars to buy votes.

Warren’s proposal is designed to buy a lot of them. One in every six adult Americans is saddled with college debt. It’s bigger than credit card debt or auto loans. Magically erasing student debt is her ploy to win over college students, adults in their 20s and 30s struggling with their loans and millions of parents left holding the bag.

Warren’s giveaway will cost taxpayers $640 billion. It’s a poke in the eye of all the assembly-line workers, waitresses, truck drivers and other taxpayers who never got to college because they went to work instead of going into debt. Why should they now have to pay someone else’s college loans?

Sure, families swamped with college debts deserve sympathy. But Democrats aren’t leveling with them or the public about what’s really causing this problem.

House Financial Services Chair Maxine ­Waters tries to pin the blame on the banking industry. When Waters had several banking CEOs appearing before her committee, including JPMorgan Chase’s Jamie Dimon and Citigroup’s Michael Corbat, she demanded their plan to fix the student loan crisis. After an embarrassing silence, they had to ­remind her banks don’t make student loans. The Obama administration took over student lending almost entirely in 2010.

In Monday’s announcement, Warren tried to vilify the federal government for “pushing families that can’t afford to pay the outrageous costs of higher education towards taking out loans.” There’s a sliver of truth to that, because Uncle Sam makes loans to virtually all comers, regardless of their ability to repay the money.

But Warren leaves out the biggest culprits — namely, the colleges themselves.

Colleges hike their tuition ­every year because they can. The feds have been willing to increase loan amounts to match whatever colleges charge.

The result? ­Every dollar lent ­inflates tuition by another 60 cents, according to Federal Reserve ­research.

In this heartless scheme, students are mules, carrying dollars from Washington to campuses nationwide. Never mind if they ever graduate or land jobs to pay back their loans.

The more money colleges rake in, the more extravagantly they spend. Federal loans help bankroll exorbitant salaries for college presidents: The University of Louisville’s president takes in nearly $4.3 million, while Columbia University’s president gets $3.9 million.

Awash in money, colleges spend way more on non-teaching staff than on faculty members. Six-figure salaries for diversity officers, admissions and financial aid managers, plus layers on layers of other administrators, all on the gravy train.

Yet all these high-priced administrators can’t manage to get students to graduate. A shocking 34% of public college students can’t make it through a four-year program even in six years. They fail, still saddled with loans to repay.

Warren wants taxpayers to forgive up to $50,000 in loans for anyone earning under $100,000 a year and smaller loan forgiveness for those earning more.

That’s the wrong idea.

Looking ahead, it would be fairer to require colleges to ­refund a portion of the loan money to Uncle Sam when a student doesn’t make it to graduation. Colleges need to have skin in the game, so they’ll try harder to get students through the course of study and into the working world.

But don’t expect to hear that kind of idea from Warren, herself a former college professor and member of the ivory tower establishment. Instead, she’s proposing more freebies. She’s offering free tuition in all public colleges and universities, partly paid for by federal tax dollars and partly by the states — a huge unfunded mandate that will hit state taxpayers hard.

Finally, Warren warns that ­under her free college plan, all public institutions must admit students regardless of their ­immigration status or criminal record. Democrats are redefining themselves as the party of ex-cons, illegal immigrants and voters who want free stuff.

Betsy McCaughey is a former lieutenant governor of New York.