WASHINGTON - Labor had hoped to send a powerful message Tuesday by taking down a centrist Democrat who strayed from its agenda.

Instead, unions were forced Wednesday to justify the estimated $10 million they spent in a high-stakes gambit that failed when Arkansas Sen. Blanche Lincoln narrowly defeated Lt. Gov. Bill Halter - labor's handpicked candidate - in a primary runoff.

It was the latest of perennial efforts by frustrated unions to convince moderate Democrats there are consequences for failing to stand with labor. But the day after, some mused that the money would have been better spent helping dozens of vulnerable Democrats in other states.

"To use $10 million during a recession on beating up their own rather than trying to save the endangered makes no earthly sense," said Doug Schoen, a Democratic political consultant.

Leon Fink, a labor historian at the University of Illinois at Chicago, said the heavy investment in Arkansas showed other Democrats that labor "will bite back if necessary." But he, too, questioned whether it was the best use of union resources.

"They turn increasingly to political campaigns when the question is whether those millions would be better put into organizing," Fink said.

Union leaders insisted that forcing Lincoln into a runoff and coming within a few thousand votes of unseating her had achieved their goal - getting wayward Democrats, who take labor's money but betray it with their votes, to think twice.

"If working families were able to accomplish this in Arkansas, imagine what they can achieve in other states," AFL-CIO president Richard Trumka said.

"It's been well worth it," said Gerald McEntee, president of the American Federation of State, County and Municipal Employees. "We've gotten more publicity and been talked about in terms of powerful labor more than I've heard in the last 20 years."

Unions have tried before to oust Democrats who voted against them on trade and other issues - with some success.

Two years ago, the Service Employees International Union helped unseat business-friendly Rep. Al Wynn of Maryland by supporting his Democratic primary opponent, Donna Edwards. In 2000, California unions rallied behind Hilda Solis to defeat incumbent Democratic Rep. Matthew Martinez in the Los Angeles area.

But those victories took place in union-friendly territory. Arkansas is a right-to-work state that ranks 49th in the percentage of workers who are union members.

Still, unions felt betrayed by Lincoln's decision to oppose card-check legislation that would make it easier for unions to organize workers. She also worked to kill a government insurance option in health-care legislation and voted against labor lawyer Craig Becker's nomination for the National Labor Relations Board.

Lincoln's turnabout on the card-check bill was particularly damaging because unions had believed she would be the 60th vote needed to defeat a GOP filibuster. Unions see the bill as crucial to ending a steady decline in union membership, which last year fell to a low of 7.2 percent of private-sector workers.

"We'll see if Blanche Lincoln is made a better senator for having to answer to working Arkansans over these past few weeks," SEIU political director Jon Youngdahl said.

Youngdahl also named a string of other Democrats who could face labor's wrath, including North Carolina Rep. Larry Kissell, Ohio Rep. Zach Space, and New York Reps. Mike McMahon and Michael Arcuri. All voted against President Obama's health-care overhaul.