Mayor Bill de Blasio, up for re-election with a double-digit lead over every rival, skipped a candidates’ forum Tuesday night in Queens, where challengers assailed him in absentia as incompetent, indifferent and corrupt.

Speaking on a stage at St. John’s University, the challengers said de Blasio has done a poor job combating homelessness, keeping the city affordable, showing up to work, relating to cops and parrying the teachers union.

“Four more years of Bill de Blasio will be an unmitigated disaster,” said Sal Albanese, who is challenging de Blasio on the Democratic ticket.

Bo Dietl said he’d like to get stuck in an elevator with de Blasio, “so I could punch him.”

“I don’t like him,” said Dietl, who isn’t running under either party because of campaign setbacks. “You know, you can’t talk violence, but I just don’t like the guy.”

Nicole Malliotakis, a state assemblywoman from Staten Island and the Republican candidate, said, “the deterioration of the quality of life across the board is a result of the bad policies that have been put forth by this administration.”

An opinion poll released by Quinnipiac University on July 31 found that de Blasio would beat every challenger: Malliotakis, 57 percent to 22 percent; if Dietl runs as an independent, de Blasio would get 52 percent of the vote, with 15 percent for Malliotakis and 11 percent for Dietl. Albanese was not included.

De Blasio faces off against only Albanese on Wednesday night in Manhattan, a debate both men are legally required to attend as beneficiaries of the city’s matching-funds program.

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De Blasio’s spokesmen did not immediately return messages seeking comment.

The trio suggested they would give raises to NYPD officers, whose rank and file have gotten 12 percent raises over five years under de Blasio. Dietl said NYPD officers should make as much as Nassau and Suffolk counties’ police forces, some of the highest paid in the nation.

The candidates assailed the United Federation of Teachers union. Dietl said, “you need a mayor that’s going to take over these damn unions, because these unions are destroying this city.” The union couldn’t be immediately reached for comment.

Moderators included Long Islanders Kayla Knight of Wantagh, a Republican, and Johnny Wiley of Uniondale, a Democrat, both 19 and St. John’s students.

The primary is Sept. 12, and the general election is Nov. 7.