Protesters march in Miami on April 15 in support of raising the minimum wage to $15 an hour as part of an expanding national movement known as Fight for 15. (Lynne Sladky/AP)

Top Democrats laid down their minimum-wage marker on Capitol Hill on Thursday, setting up their party's middle-class-focused economic message heading into the 2016 elections campaigns.

Their pitch: "$12 by '20" — a $12 per hour federal minimum wage by 2020, which they say will give a pay raise to nearly 38 million Americans.

"This is a key piece of our effort to grow the economy from the middle out not from the top down," said Sen. Patty Murray (D-Wash.), a lead sponsor of new legislation to reset the minimum wage. "It would help the economy grow in a sustainable way, and it’s an important step toward expanding economic security and making sure more families can make ends meet."

The federal minimum wage was last raised in 2007, when Congress agreed to bring it from $5.15 to $7.25 over three years. The $12 an hour wage, supporters say, will restore the purchasing power that the federal minimum wage had in the late 1960s, when the American manufacturing economy was at its height, and provide enough income to keep a family of three out of poverty. As of Jan. 1, 29 states already had minimum wages higher than the federal minimum.

[Jan. 1: Twenty states raised their minimum wage today]

But Republicans, with a few exceptions, have grown consistently move opposed to minimum wage increases in recent years, arguing that any hike would tamp down economic growth. Last year, for instance, House Speaker John Boehner (R-Ohio) said he considered any minimum wage hike to be "bad policy" that would hurt minorities and others holding low-wage jobs by eliminating some of those jobs.

"When you raise the cost of something you get less of it," he said after President Obama announced an executive order raising the minimum wage for federal contractors, adding, "The very people the president purports to help are the ones who are going to be hurt by this."

Labor Secretary Thomas Perez, appearing with top congressional Democrats, said the wage bill introduced by Murray and Rep. Bobby Scott (D-Va.) in their respective houses carries "the full and enthusiastic support" of Obama, who has repeatedly called for a higher minimum wage. "This federal floor is an indispensable part of who we are as a nation," Perez said.

"We need to honor work," Scott said. "This means that people who do work shouldn't have to live in poverty."

With the GOP in control of both houses of Congress, there is little expectation that the Raise the Wage Act will be taken up in the next 20 months. But the bill will be a rallying point for Democrats on the campaign trail — not only congressional candidates, but probably for the party's presidential hopefuls as well.

Under the new legislation, the increase from $7.25 to $12 would take place over five years, starting with an $8 wage and increasing by $1 per year through 2020. After that, the wage would be indexed to the growth in the national median wage. The bill would also gradually eliminate the lower "subminimum" wage for tipped workers.

[The Raise the Wage Act: The new proposal to raise the federal minimum wage to $12 in 2020]

The $12 figure falls short of the $15 minimum wage that was recently mandated in Seattle and which labor unions and activists have called for nationwide in recent months.

But Democratic leaders said Thursday that the $12 by '20 goal was a realistic and achievable goal that they feel Democratic candidates will have no trouble embracing going into 2016 — highlighting, in Perez's words, "remarkable differences" between the parties.

"I want to hear what the Republican presidential candidates have to say on this," Murray said. "I'm confident that a Democratic woman running for president and anyone else who chooses to knows the importance of this issue and helping lift American workers to a better place. I am not at all confident there is one Republican presidential candidate who will state the same."