For all the sniffing over various campaign no-no’s in this year’s mayoral race, the biggest one of all has passed without notice: The Working Families Party’s open efforts to decide the Democratic Party’s primary.

This is far different from a cross-endorsement in the general election, far closer to subversion or a hostile takeover.

Yes, everyone in local politics knows the WFP was basically founded to drag the Democrats left, but that was supposed to be via the same outside pressure the state Conservatives have deployed on the Republican Party.

But the WFP’s official announcement of its ranked-choice preferences in the Democratic primary crosses a clear line: It’s one thing for a Democratic faction (such as the Democratic Socialists) to endorse, another for an outside group to do so.

The laws say only a party’s registered voters get to decide, yet an outside party is announcing its plan “to consolidate support around its slate of candidates.”

Even though the WFP has its own ballot line in November’s general election.

As we noted in April, this takeover is further empowered by the city’s cockamamie public-campaign-finance and ranked-choice-voting laws, even though those “reforms” were sold as boosting outsiders.

Campaign-finance-board rules don’t even require the WFP to disclose what it’s spending to influence the Dems’ primary!

Somehow, the “closed” Democratic primary has become an open one, with the WFP privileged to interfere with zero accountability.

The WFP complains that “the American political system is rigged to favor the two major parties,” but it sure looks like New York’s system is rigged to favor the WFP.

Some Democrats have long questioned the WFP’s involvement in Dem primaries, but party leaders shut down any discussion because they benefit from the corrupt process.

Now that the violations are careening out of control, maybe they’re having second thoughts.