Like a dormant volcano, the simmering mayoral election battle between Republican Sam Katz and Democrat John Street unexpectedly boiled over yesterday in a heated war of words over a national labor leader's surprise broadside at Katz.

The sharp exchange came at a press conference where the AFL-CIO umbrella labor group endorsed former City Council chief Street.

United Mine Workers head Rich Trumka, who hails from West Virginia, blasted the GOP's Katz as an elitist who is out of touch with working people and who would seek to privatize government functions like the Pennsylvania Turnpike and state liquor stores.

Trumka's verbal blast drew a hastily drafted letter from Katz's campaign manager, Bob Barnett, to Street, asking the Democratic nominee to repudiate the union chief's comments. Barnett accused Street of resorting "to a low level campaign of cheap shots by your surrogates."

Caroline Brobeil, Street's campaign spokeswoman, responded last night that while Katz has never held elected office before, he has called in past campaigns for privatizing functions like the turnpike, liquor stores and even city trash collections.

"I wonder if Katz is saying now that he didn't mean it - that doesn't exactly inspire public confidence," Brobeil said. "As a candidate, you have a record and you have to live with it, and apparently he wants to wipe the slate clean."

The flap over Trumka's comments obscured a key labor endorsement that is something of a coup for Street, since he ruffled a few feathers in the organized labor movement for helping Mayor Rendell win municipal union givebacks when he was City Council president in the early 1990s.

The endorsement of the Philadelphia Council AFL-CIO included a $5,000 campaign contribution but more important will place a small army of field workers and phone-bank staffers at Street's disposal as the fall election draws closer.

Although the AFL-CIO historically has backed the Democratic mayoral nominee in Philadelphia - where Democrats outnumber Republicans by a 4-1 margin, even council leader Joe Rauscher conceded that "it was not an easy task" for the group to decide on Street and a slate of Council and judicial candidates.

Insiders said yesterday some union groups, mainly representing city employees and hospital workers - many of whom backed Street's top rivals in the May Democratic primary - had lobbied privately against endorsing Street because of his record.