NEW YORK -- Former vice- president Al Gore was to endorse Howard Dean for the Democratic presidential nomination Tuesday, a breakthrough on the eve of the primary season that could tighten Dean's grip on the front-runner's position and usher more support from wary party elite.

Gore, who won the popular vote but lost the electoral vote in the disputed 2000 election, has agreed to appear with Dean in New York City's Harlem neighborhood and then travel with the former Vermont governor to Iowa for a formal endorsement, several Democratic officials, including one close to Gore, said Monday.

Five weeks before Iowa's kickoff caucuses, the coveted endorsement is a breathtaking victory for a candidate whose anti-war, anti-establishment candidacy has given pause to party leaders and key constituencies, several Democratic strategists said.

"What this says is that all these Washington insiders who have been gnashing their teeth, wringing their hands and clinging to their cocktail cups can relax now.

Dean's been knighted by the ultimate insider," said Democratic consultant Dean Strother of Washington. "It's game, set and match. It's over." Other Democrats offered more cautious appraisals, but the overwhelming consensus was that Dean's coup makes him the overwhelming favorite to claim the nomination.

Even advisors to Dean's rivals conceded they were stunned and disheartened by the news.

Gore won the popular vote by half a million votes in 2000 but conceded to Republican George W. Bush after a tumultuous 36-day recount in Florida and a 5-4 Supreme Court vote against him. Gore's concession came Dec. 13, 2000.