Mayor de Blasio’s poll numbers have tumbled — but the lower favorability ratings haven’t come close to derailing his bid for re-election in November, according to a poll released Monday.

Voters approve of the mayor’s job performance by a 50 to 42 percent margin — down from a 60 to 34 percent showing on May 17, according to the Quinnipiac University survey.

They are also split on whether he deserves a second term — 46 to 46 percent — even though 57 percent had supported his re-election bid in May, while 35 percent did not.

But when it comes to a general election match-up against Nicole Malliotakis, a Staten Island Assembly member, de Blasio is still up 57 percent to 22 percent.

His lead in May was 64 to 21 percent.

“Leave out the one big question – the re-election matchup – and this is a pretty bad poll for Mayor Bill de Blasio,” said pollster Maurice Carroll.

“But on the one question that matters in this election year — de Blasio versus Assembly member Nicole Malliotakis — Hizzoner is a better than a 2-1 winner.”

When independent candidate Bo Dietl is factored in, de Blasio’s margin of victory gets even larger — 52 percent for the mayor, 15 percent for Malliotakis and 11 percent for Dietl.

The issue for Malliotakis and Dietl appears to be largely name recognition.

When it came to favorability ratings, 78 percent of voters said they didn’t know enough about Malliotakis to form an opinion — while 74 percent said the same of Dietl.

One of the highest numbers registered in the poll came when voters were asked who should pay the mayor’s legal bills for state and federal investigations of his administration.

Seventy-eight percent said he should cover the $2 million, while just 13 percent say taxpayers should pick up the tab.

De Blasio initially said he’d raise the money in a legal defense fund — then changed his mind and said he’d stick taxpayers with the bill.

The poll surveyed 877 city voters between July 20 and July 26, and has a plus or minus 4.1 percentage point margin of error.