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Speaking before hundreds of striking federal contract workers outside the Capitol, Senator Bernie Sanders of Vermont unveiled legislation Wednesday that would raise the national minimum wage to $15 an hour.

Mr. Sanders, who is seeking the Democratic nomination for president, and other members of the Congressional Progressive Caucus at the rally also pressed President Obama to take executive action that would give preference in awarding federal contractors to companies that pay at least $15 an hour and give their workers the right to unionize.

“Today, we send a very loud and clear message to the United States Congress, to the president of the United States and to corporate America,” Mr. Sanders said. “In the richest country on the face of the earth, no one who works 40 hours a week should be living in poverty.”

The federal minimum wage is currently $7.25 an hour. Mr. Sanders’s proposal would raise it incrementally until 2020, when it would hit $15.

Mr. Sanders, who identifies as a socialist, has made wage growth a key tenet of his campaign, consistently trumpeting the $15-an-hour minimum wage in stump speeches across the country.

A more modest proposal by Senator Patty Murray, Democrat of Washington, to raise the national minimum wage to $12 an hour won the backing of the Obama administration and several key congressional Democrats this spring. But with Republicans in control of Congress, the proposal gained little traction.

Representative Keith Ellison, Democrat of Minnesota, and Representative Raúl M. Grijalva, Democrat of Arizona, said they would introduce parallel legislation in the House.

Hillary Rodham Clinton, Mr. Sanders’s main rival for the nomination, has been more reticent in endorsing the $15-an-hour minimum wage, even as she has indicated some support for the effort.

“I support the local efforts that are going on that are making it possible for people working in certain localities to actually earn 15,” Mrs. Clinton told reporters in New Hampshire this month.

Speaking with reporters Wednesday after the rally, Mr. Sanders declined to address Mrs. Clinton’s position.

“That’s her view,” he said. “I support 15 an hour. She can do what she wants.”

Former Gov. Martin O’Malley of Maryland, another Democratic contender, supports the $15-an-hour minimum wage.

A February 2014 executive order raised the minimum wage for federal contract workers to $10.10 an hour.