Mayor Bill de Blasio has repeatedly touted the “grassroots” nature of his quixotic presidential campaign but nearly 70 percent of his donors are tied to a single union — the powerful Hotel Trades Council.

The 40,000-member HTC is the only union that’s endorsed de Blasio’s long-shot 2020 bid.

The June 5 endorsement came after his administration’s crackdown on union rival Airbnb.

De Blasio’s administration subpoenaed Airbnb to hand over listing information as part of its crackdown on illegal rentals and created an Office of Special Enforcement to investigate unlawful sublets.

And union president Peter Ward also praised the mayor for requiring hotel developers in certain areas to get special permission for their projects.

“In just a few short years we changed the zoning in this city thanks to this mayor to create a fair process to make sure that hotels that come here to operate go through a process that guarantees they serve not just their interest but the communities,” Ward said.

In the subsequent weeks, some 4,600 hotel and hospitality workers gave a total of $89,000 to the campaign with the average donation being just under $20, according to the Center for Public Integrity campaign finance data reviewed by The Post.

As of June 30, he’d secured $1.1 million from a total of 6,700 donors.

Shortly after launching his long-shot White House bid in May, de Blasio told NY1 his campaign would have a “grassroots-focused approach.”

A spokeswoman for the de Blasio campaign did not return a message and an HTC rep declined to comment.