The Metro Washington Council AFL-CIO, D.C.’s largest labor group, voted last week to support mayoral candidate Muriel Bowser. (Bill O'Leary/The Washington Post)

A key element of the District’s Democratic political establishment rallied Monday behind mayoral candidate Muriel Bowser as the top leader of the city’s largest labor group warned about the perils of electing a former Republican to the office.

The Metro Washington Council AFL-CIO voted last week to endorse Bowser, said Joslyn N. Williams, the council’s longtime president. He cited her vote in favor of a higher minimum wage for D.C. workers, her backing of union-friendly “project labor agreements” on city-financed projects, and her support for various organizing efforts across the city.

Bowser, the D.C. Council for Ward 4, accepted the endorsement Monday morning in front of the labor organization’s national headquarters downtown. She called it a “big boost” to her campaign in the face of energetic challenges from two former D.C. Council members running as independents.

She sought to draw contrasts with one of her rivals, fellow council member David A. Catania (I-At Large), citing her recent support for an effort to organize employees of a D.C. construction company and her vote in favor of a 2008 bill that mandated paid sick days for city workers.

“Now who could be against that?” she asked. “The opportunity for people to stay home with a sick child or stay home themselves if they became sick?”

Catania, a former Republican running as an independent, voted against the 2008 bill on a 7-to-6 first vote and then supported the bill on a unanimous second and final vote. Last year, he introduced legislation to expand the sick-leave law; he voted for a similar bill that became law this year.

Another mayoral candidate, Carol Schwartz, shepherded the 2008 bill through the D.C. Council; opponents of the bill were among those who supported a successful challenge to the four-term veteran in the Republican primary that year. Schwartz, too, is now running as an independent.

Williams recently spoke fondly of Schwartz’s efforts on the sick-leave bill, but he said her efforts six years ago were not enough to win her the AFL-CIO endorsement, which came after each candidate completed a lengthy questionnaire and participated in an interview with labor leaders.

“In our pecking order, it was Bowser, Schwartz, Catania,” he said. “But we just didn’t think Carol Schwartz was a viable candidate.”

Williams had harsher words for Catania, who has positioned himself as being more liberal than Bowser on some issues. Catania “cannot be relied on to be worker-friendly,” he said, adding that Catania’s election would be “the same as it would mean for labor to have a George W. Bush” as president.

“He may be progressive on certain social issues . . . but in this city, you can’t get elected unless you have certain social values,” Williams said. “On economic issues, we differ with him and his vision of the economy for the city.”

Ben Young, Catania’s campaign manager, declined to comment on Williams’s characterizations.

Bowser said she planned to demonstrate her labor bona fides in the coming weeks by working a shift as a housekeeper at a downtown hotel “to see how their life is, to see how hard work affects how they can support their families.”

The labor council’s endorsement for Bowser comes after it made none in the Democratic primary, with member unions split between Bowser, incumbent Vincent C. Gray and other candidates. Monday’s event was a show of solidarity that included leaders of several unions that backed Gray in the primary.

They included Andre Lee of the American Federation of State, County and Municipal Employees, who said his union was committed to Bowser despite its earlier support for Gray.

“Either you play on the field, or you sit on the sideline,” Lee said. “And I’m a ballplayer.”