What can de Blasio do with his leftover campaign cash? It’s complicated.

NEW YORK — Former Mayor Bill de Blasio ended his congressional bid, and his political career, Tuesday with $450,000 left in campaign coffers and a pile of unrelated debt — now he may be able to use the leftovers to fulfill some of his outstanding obligations.

The Park Slope resident has lingering debt with other campaign accounts and owes six figures to a white-shoe law firm. The city’s Department of Investigation has also hit him with the tab for a city-funded security detail during his 2019 run for president.

Rules from the Federal Election Commission, however, limit what de Blasio can do with the cash. He cannot, for instance, appropriate the money for personal use.

The backstory: The former mayor’s obligations stem from a few sources.

A federal political action committee he set up in 2018 called Fairness PAC owes $30,625 in outstanding payments but has just over $3,000 on hand.

His presidential campaign account is similarly underwater by about $75,000.

De Blasio has vowed to set up a legal defense fund to pay more than $300,000 to Kramer Levin Naftalis & Frankel, which represented him during an ethics investigation into his fundraising. The city's Conflicts of Interest Board said Wednesday that no such fund has been set up.

And the city’s Department of Investigation has called on the mayor to pay around $320,000 to cover the cost of his NYPD security detail during his presidential run.

A debt-free BdB? The mayor could keep the money at the federal level to use for a future run for office or for a political action committee, but the FEC generally prohibits candidates from transferring leftover cash back to old accounts, a spokesperson said Wednesday.

That means it’s unlikely the mayor would be able to wipe out what he owes from his federal PAC. Instead, he would be limited to contributing $5,000 annually.

However, an advisory opinion from 1987 pertaining to the ill-fated presidential run of late Sen. John Glenn suggests that de Blasio may be able to pay off debts stemming from his own quest for the White House. And according to the FEC, there is no rule prohibiting campaigns from reimbursing localities for the cost of security — meaning his tab with the NYPD could be fair game.

De Blasio could also transfer the money wholesale to a state committee, according to the FEC and the state Board of Elections. He would have two to choose from: The New York Fairness PAC, which he ostensibly set up to support the campaigns of like-minded Democrats, and New Yorkers for a Fair Future, the vehicle he used to explore a run for governor last year.

Both of those committees are solvent.

Election lawyer Martin Connor said the 2017 ethics probe pertained to de Blasio's fundraising activity on the state level, so he might be able to transfer the campaign cash to a state committee and then put it toward his tab with Kramer Levin.

"[The investigation] did relate to his political activities in the state, and/or his actions as a public official in the state," Connor said. "So I think he can use state campaign funds to pay that bill."