WASHINGTON — Tuesday's Wisconsin primary is shaping up to be a critical test of whether Hillary Rodham Clinton can hold on to her base of support among low-wage and blue-collar workers despite the growing number of labor unions endorsing Barack Obama.

The Service Employees International Union — which represents 150,000 healthcare workers, janitors, security guards and municipal employees in the upcoming primary states of Wisconsin, Texas, Ohio, Pennsylvania and Rhode Island — announced Friday it will campaign for Obama.

For Clinton, who has stressed that Obama's healthcare plan does not require universal coverage as hers does, it's a major setback because the service employees' union has been one of the most vocal advocacy groups for universal coverage.

"It speaks to the constituency that Obama needs, which is low-income immigrant Latino workers," said Jefferson Cowie, an associate professor of labor history at Cornell University.

Both candidates are honing their messages to appeal to blue-collar workers.

Obama — who has done well among higher-income voters, blacks and young adults — delivered what his campaign described as a major economic address Wednesday at a General Motors factory in Janesville, Wis.

Clinton, whose voter base has been people over age 50, rural whites and voters with incomes under $50,000, spent most of the week campaigning in Texas and Ohio. Her first stop in Ohio was a factory in Youngstown. On Friday, she visited a Lockheed Martin plant in Akron. And earlier in the week, she visited a factory in Maryland that makes transmissions for hybrid vehicles.

Andy Stern, president of the service employees' union, said reporters in a conference call that he thinks there's very little difference between Clinton and Obama on issues important to organized labor.

"This is really not about specific positions alone," Stern said. "It's about the right person at the right time. And we think this is a moment where there is an opportunity for fundamental change in this country."

On Thursday, the United Food and Commercial Workers union, representing supermarket and food processing employees, also endorsed Obama.

"I think her firewall is beginning to break down," said James Thurber, director of American University's Center for Congressional and Presidential Studies.

Thurber pointed to the recent endorsement of Obama by the activist group MoveOn.org and by Sen. Edward Kennedy, D-Mass.

"It really solidified the left of the party and said this is the guy we want," he said.

Clinton continues to have the longstanding backing of nearly a dozen national labor unions representing building trades and construction workers, machinists and public employees, but her recent string of presidential primary losses has made it imperative that she register blowout victories in the upcoming primaries to whittle down the lead Obama has in pledged delegates to the Democratic National Convention.

While John Edwards remained in the race, many labor unions were deeply split among the three candidates, with Clinton and Edwards capturing most of the endorsements and many unions choosing to stay on the sidelines.

The two recent endorsements give Obama the support of three of the seven unions in the Change to Win labor federation. The third — UNITE HERE — represents workers in textile factories, laundries, restaurants and hotels.

However, a fourth member of Change to Win — the United Farm Workers — endorsed Clinton last month and helped her win the California primary on Super Tuesday.

Wisconsin and the March 4 primary states of Ohio and Rhode Island all have a greater concentration of unionized workers than the national average.

Change to Win plans to decide this week whether to have its unions back Obama as a group.