Mayoral challenger Lori Lightfoot made a splash last month — and planted her flag as a lead competitor — by raising $243,000 for her campaign right out of the gate.

But that’s a drop in the bucket in comparison to the big bucks that Mayor Rahm Emanuel is piling up.

Emanuel has flexed his legendary fundraising muscle to the tune of $890,000 in the last few days alone.

That leaves his campaign fund with roughly $6.5 million for, what’s expected to be his most difficult race against nine challengers.

Emanuel still has a long way to go to match the $24.4 million he raised and spent to survive Chicago’s first mayoral run-off against Jesus “Chuy” Garcia in 2015. But he’s getting there.

This week’s mayoral donor drop includes big money from organized labor.

Heavy-hitters include: $250,000 from the Engineers Political Education Committee and $50,000 apiece from Teamsters Local 731 PAC, Teamsters Volunteers in Politics PAC and the International Union of Operating Engineers Local 399 PAC.

Other big donations came from: John Arnold ($200,000); Bryan Cressey ($100,000); Kirkland & Ellis Chairman Jeffrey Hammes ($50,000); Howard Gottlieb ($50,000) and White Sox and Bulls owner Jerry Reinsdorf ($30,000).

Other Kirkland & Ellis attorneys also contributed $110,000. Fox Ventures LLC kicked in $25,000.

With fundraising limits for all mayoral candidates lifted by millionaire businessman Willie Wilson’s $100,000 contribution to himself, Emanuel was once again free to tap his deep reservoir of high-rollers.

The mayor once again showed why his fundraising Rolodex is the envy of Democrats nationwide.

The heavy support from trade unions was expected after Emanuel hammered out a recent contract with the building trades that guaranteed labor peace, reduced health care costs and preserved the prevailing wage paid to their counterparts in private industry.