Alarmed by Eliot Spitzer’s momentum in his unexpected bid to win citywide office, an unlikely coalition of business leaders, women’s groups and labor unions is vowing to finance an ambitious effort to thwart the former governor’s ambition.

The interest groups, which often spar with one another over competing agendas and priorities, have found rare common cause in their antipathy toward Mr. Spitzer, who infuriated the business community with his aggressive posture toward Wall Street, who offended feminists by paying for sex with prostitutes and who alienated unions by taking on a labor-backed candidate.

Now, they are pledging to raise and spend at least $1.5 million on advertising, direct mail and field work in an effort to persuade voters that Mr. Spitzer would be a poor choice for comptroller, New York City’s chief financial officer. That money may be dwarfed by the amount Mr. Spitzer, whose family has a real estate fortune, could spend on his own, but is enough to alter the contours of the campaign, particularly because the groups are likely to be more negative than Mr. Spitzer’s Democratic primary opponent, Scott M. Stringer, the Manhattan borough president.

Taking a cue from the changing landscape of presidential politics, the groups are forming two “super PACs” that will be free to accept contributions of any size, unhindered by the restrictions that limit gifts to Mr. Stringer’s campaign.