Several federal workers and supporters march from Freedom Plaza to Lafayette Park near the White House on Wednesday as part of a demonstration protesting low wages. (Michael S. Williamson/The Washington Post)

After 19 years cleaning floors and bathrooms in Union Station, Vilma Martinez still earns only $8.25 per hour. A few blocks away at the National Museum of the American Indian, Calvin Bolton washes dishes in the cafeteria for $9.44. When business is slow, both workers said, their hours are cut back with almost no warning.

“I’m really struggling. My daughter’s 17, and I feel awful that I can’t afford to get her what she needs,” said Bolton, 57, who joined a protest rally Wednesday outside the White House, demanding higher wages for federally contracted low-wage workers. “President Obama has two daughters,” he said. “He should know how this feels.”

About 250 Hispanic and African American workers from half a dozen federal sites in the District marched from Freedom Plaza to Lafayette Square. They carried stacks of boxes representing 250,000 petitions to Obama and gigantic pens with a message urging the president to sign an executive order guaranteeing them a “living wage.”

Organizers at the march, sponsored by several labor and religious groups, said they were representing about 90,000 low-wage federal workers in the Washington area, among some 2 million nationwide. They described the workers as an “invisible army,” saying that they toil in iconic federal facilities and tourist sites but are employed by private contractors with no health benefits, vacation or sick pay.

The workers’ campaign, which has been largely organized by an advocacy group called Good Jobs Nation, has gained momentum and visibility in recent months, chiefly through a series of one-day public actions. In May, workers picketed outside the Ronald Reagan Building and several other federal work sites. In July, they staged a day of protests, strikes and street theater outside several Smithsonian museums. The group has also filed a complaint with the U.S. Department of Labor accusing private vendors in the Reagan Building of “wage theft.”

On Wednesday, demonstrators converged outside the White House, chanting slogans in English and Spanish and were accompanied by Christian, Jewish and Muslim clerics. Many chanted, “We can’t survive on $8.25.” Organizers said workers who attend the one-day rallies are paid a stipend to cover transportation and meals.

At noon, a delegation of six workers was escorted into the White House for a meeting with several officials. They told their personal stories of economic hardship and delivered a flash drive containing all 250,000 signatures. They also brought a letter of support from 15 senators, including Bernard Sanders (I-Vt.), who briefly addressed the rally.

The senators’ letter urged Obama to set a special minimum wage of $10.10 per hour for federal contract workers, higher than the current federal minimum wage of $7.25 and the District minimum of $8.25. “Profitable corporations that receive lucrative contracts from the federal government should pay all their workers a decent wage,” the letter said. A proposal by Senate Democrats to raise the national minimum wage to that level has been blocked by Republicans.

“The officials we met with asked good questions. They were clearly moved by the stories they heard, and they seemed very receptive to the concerns we shared,” said the Rev. Michael Livingston, an official of the Chicago-based Interfaith Worker Justice movement, who also attended the White House meeting.

There was no public comment Wednesday from the White House. In the past, officials have said that federal contractors set their own wages and hours but that they are expected to generally follow federal employment guidelines.

The mood at the rally was upbeat and boisterous, except for a few reflective moments when the marchers sat down at an intersection along New York Avenue, surrounded by police escorts, and sang several verses of “Amazing Grace.” There were young black women with baby strollers, Central American grandmothers, and a crowd leader with a bullhorn and a white Uncle Sam-style beard.

“A lot of people work in the same places and we realize we are all in the same situation,” said organizer Tara Young. “We know who the real enemy is, and it is not each other.”

When asked about their lives, several workers offered grim glimpses of their struggles to survive on one of the most precarious rungs of the economy. Jose Hernandez, 25, who works in a fast-food restaurant at the Smithsonian, said he takes home $200 a week and spends it all on rent and on money he sends to his mother in El Salvador.

Victor Harold, 55, a resident of Riverdale, said he earns $10.70 an hour as a short order cook at a concession in the National Museum of American History. Like many others at the rally, he said his work hours had been cut substantially since the summer visitor rush ended. At the end of the week, he said, “you never have anything left.”