Washington ? In a major boost for the presidential candidacy of former Vermont Gov. Howard Dean and a body blow to one of his closest rivals, Dick Gephardt, two of the country’s biggest labor unions signaled Thursday they’re ready to jump on Dean’s bandwagon.

The 1.6 million-member Service Employees International Union effectively endorsed Dean Thursday but postponed its formal announcement until Wednesday, only because it was asked to by the 1.4 million-member American Federation of State, County and Municipal Employees — which is then expected to join it in backing Dean.

The double-barreled boost will give front-runner Dean crucial money and manpower in key Democratic early-voting states, especially Iowa, where Democrats vote Jan. 19 and where Dean and Gephardt are running neck and neck. The embrace by labor unions also shows that Dean’s appeal extends beyond the well-educated liberals who have dominated his following so far.

Gephardt may be hurt even more than Dean is helped by the 1-2 union punch.

The veteran congressman from Missouri has built his career and presidential campaign on a foundation of loyalty to labor, and the rejection of his candidacy by two major unions suggests that even those who know him best lack confidence in him.

“This doesn’t necessarily secure the nomination for Dean, but it shoots an arrow into a vital organ of the Gephardt campaign,” said Dennis Goldford, a political scientist at Drake University in Iowa.

Labor unions help provide the kind of organization that’s crucial to getting people to attend Iowa’s caucuses.

Democratic presidential hopeful former Gov. Howard Dean of Vermont, center, smiles as he joins the leadership of the Service Employees International Union. The union is expected to endorse Dean's candidacy next week.