The mayor’s race heats up this evening with a big showdown on tap at the Labor Temple in Belltown.

Chances are good that the King County Labor Council, which represents 75,000 union members in King County, will make a formal endorsement for the Nov. 5 general election.

“If I were to handicap it, I’d say Mayor McGinn is in a stronger position to get the two-thirds needed,” Labor Council executive secretary David Freiboth told Seattle Weekly this morning. “I’m not sure Sen. [Ed] Murray will get the two-thirds support.”

Much is riding on this endorsement. It means money, direct mail, and phone-banking on behalf of the favored contender.

Frieboth predicts that as many as 100 delegates from the council’s affiliates will show up for what promises to be a spirited debate.

How much impact a union endorsement will have in this race remains to be seen. Four years ago, the Labor Council backed two losers – throwing its support to then-Mayor Greg Nickels in the primary. When Nickels finished third, they chose to support Joe Mallahan over McGinn, who alienated the building and trades wing of the Labor Council’s membership for opposing construction of the waterfront tunnel.

McGinn has shored up his support with organized labor in recent years.

Freiboth said it is doubtful that the Council would consider a dual endorsement.

“No, I think we should weigh in on this race and back a candidate,” said Frieboth.