Republican politicians such as Jeb Bush, left, Mitt Romney and Sen. Marco Rubio have been putting issues of middle-class wage stagnation, poverty and shared prosperity at the forefront of their political messages. (Melina Mara/The Washington Post)

Presidential hopefuls in both parties agree on at least one thing: Economic mobility, and the feeling of many Americans that they are being shut out from the nation’s prosperity, will be a defining theme of the 2016 campaign.

Former Florida governor Jeb Bush last week became the latest Republican to signal a readiness to engage Democrats on what historically has been their turf, putting issues of middle-class wage stagnation, poverty and shared prosperity at the forefront of their political messages.

Bush’s framing of the economic and social challenges facing the country nearly mirrors that of likely Democratic candidate Hillary Rodham Clinton, as well as other possible contenders on the left. Sen. Marco Rubio (R-Fla.) has written a book on the subject, “American Dreams: Restoring Economic Opportunity for Everyone,” to be published this week, while Sen. Rand Paul (R-Ky.) has proposed policies for distressed communities that he sees as “the ticket to the middle class.”

And Mitt Romney, the 2012 GOP nominee who was portrayed by Democrats as insensitive to and out of touch with the lives of middle- and working-class Americans, has told friends he considers poverty a topic du jour as he weighs another run in 2016.

“You talk to any pollster, on the Democratic side or the Republican side, they’re in complete agreement on the idea that there has to be an economic populist message,” said Matthew Dowd, a top strategist for former president George W. Bush’s 2000 and 2004 campaigns. “Then it comes down to ‘Are there credible solutions and is there a credible candidate?’ ”

About 45 million Americans live at or below the poverty line, according to last fall’s census estimates, while the median household income in the United States in 2013 was just under $52,000. Adjusted for inflation, the median is 8 percent lower than it was in 2007, the last full year before the recession, and 11 percent below what it was in 2000.

Wage stagnation has been a persistent problem for low- and middle-income workers. “Since the late 1970s, wages for the bottom 70 percent of earners have been essentially stagnant, and between 2009 and 2013, real ­wages fell for the entire bottom 90 percent of the wage distribution,” Lawrence Mishel of the liberal Economic Policy Institute wrote in a paper published this month.

Launching his new political action committee, Right to Rise, Jeb Bush asserted last week in the PAC’s mission statement that President Obama’s tenure has been “pretty good” for those at the top of the income scale but a “lost decade” for everyone else.

“Millions of our fellow citizens across the broad middle class feel as if the American Dream is now out of their reach; that our politics are petty and broken; that opportunities are elusive; and that the playing field is no longer fair or level,” Bush wrote. “Too many of the poor have lost hope that a path to a better life is within their grasp.”

Bush’s focus adds an unexpected element to the coming debate and puts pressure on the entire field, himself included, to come forward with fresh policies that address the nation’s core economic problems.

The shift also highlights a potential vulnerability for Democrats. In his campaigns, Obama twice promised to focus on wealth inequality and middle-class stagnation, but those problems persist even as the economy revs up and unemployment falls.

Clinton could be challenged now from both the right and the left to lay out solutions that go beyond Obama’s — and also to develop a political voice of her own to articulate them. Democrats conceded after last fall’s midterm elections that they had not found a compelling economic message.

Democrats will look to Clinton, should she run, to take the lead in doing that — although they also are looking to Sen. Elizabeth Warren (D-Mass.) and her harder-edged populist rhetoric.

Clinton tested such a message last year. In a major speech at the New America Foundation in May, she said that Americans are “finding it harder than ever to get their footing in our changing economy. The dream of upward mobility that made this country a model for the world feels further and further out of reach.”

The language of Bush, Rubio and other Republicans appears aimed at avoiding the problem that bedeviled Romney throughout his 2012 campaign. The former Massachusetts governor’s memorable comment about the “47 percent” of the population who he said depend on government and feel like victims haunted him during the final months before the election.

Yuval Levin, editor of the conservative journal National Affairs, said the overall shift in emphasis by Republicans is an effort to move away from Romney’s more abstract message of job growth and to focus more specifically on social mobility and solutions for those at or near the bottom.

“It’s a process of letting the light out from under the bushel,” Levin said. Citing past conservative leadership in reforming welfare, he added: “I think Republican ideas have always held this promise, but conservatives have not always emphasized these elements. . . . A difference in emphasis is not insignificant.”

Democrats, however, argue that revising their rhetoric will not be enough for Republicans — especially if Jeb Bush becomes the GOP nominee — to gloss over former president George W. Bush’s legacy, which Democrats say tightened the middle-class squeeze.

“It’s not like the Republican Party has clean hands on the issue of rising inequality,” said Neera Tanden, president of the Center for American Progress and an adviser to Clinton on economic policy. “It’s a weird thing to say, ‘I care about how the rest of the country is doing, not just the top earners,’ when your brother cut taxes for the wealthy and your party’s economic position starts with undoing Dodd-Frank,” the 2010 law that tightened regulation of Wall Street.

Obama hit the road last week to highlight economic successes of his presidency, from the auto industry bailout to more robust overall growth in the last half of 2014. Still, many Democrats acknowledge that there is a large gap between promise and results on issues of middle-class insecurities.

That is where Bush and some other Republicans are taking aim. Two factors have contributed to the opening they see.

One is the continued problem of stagnating wages for the broad middle class and the income gap between rich and non-rich. The other is new evidence that it has become harder for those at the bottom to rise into the middle class, and that the risk of some born into the middle class — particularly minorities — falling out of it is growing.

Democrats long have been seen as the party more trusted to deal with middle-class economic issues. Republicans long have resisted policies that smack of economic redistribution. Could that be changing?

“How do you tell the middle class, ‘We’re your guy?’ ” said Grover Norquist, a conservative anti-tax advocate. “Republicans feel comfortable saying that now because they feel the guys they’re running against are sufficiently discredited.”

William Galston of the Brookings Institution said that the issue of blocked social mobility is one Republicans will feel more comfortable engaging — and on their own terms. “That’s what Jeb Bush is saying: ‘We can accept a definition of a problem . . . but give unabashedly conservative responses to those challenges,’ ” he said.

But Galston, a former domestic policy adviser to President Bill Clinton, said Bush’s framing of the issue this way means he has put down a marker that he will be expected to meet.

“I would expect to hear from him over the coming months some of the best and most innovative conservative thinking on opportunity and upward mobility,” he said. “That will be a fair test on how much gas there is in the conservative ideas tank.”

Other Republicans have been working to reorient their party toward blue-collar economics and to put forward fresh economic ideas. Former senator Rick Santorum of Pennsylvania and former Arkansas governor Mike Huckabee, two former presidential candidates who are looking at running in 2016, long have talked about the GOP’s need to recognize the problems of working families.

House Ways and Means Committee Chairman Paul Ryan (R-Wis.) has focused on policies designed to lift people out of poverty. In 2012, as Romney’s running mate, Ryan wanted to campaign in inner cities and give speeches promoting individual empowerment. He was frustrated that Romney’s campaign team directed him to talk about other topics.

Rubio has proposed a fundamental shift in anti-poverty programs. He has suggested devolving power and responsibility from Washington to the states by consolidating federal monies into a “flex fund.” The senator from Florida has called for significant reductions in federal regulations that he says will spur creation of better jobs. On education, he has offered a series of reforms designed to lessen the burden of student loan debt and expand access to apprenticeship programs. Rubio and others also have proposed changes in the earned-income tax credit to extend its reach to more people.

Democrats are skeptical that Republicans can meet the challenge with policy proposals that are much beyond calls for tax cuts to spur economic growth and further efforts to scale back the size of government.

“It sounds to me like a ­traditional Romney Republican ­trickle-down agenda but with a willingness to engage on inequality and mobility,” said Jared Bernstein, a former adviser to Vice President Biden who works at the Center of Budget and Policy Priorities.

Democrats say it’s too early to draw conclusions, but they note that if Bush and other Republicans find new ways to engage middle-class voters with populist themes, the pressures on Clinton will mount significantly.

“Voters are going to [say], ‘Okay, thank you for acknowledging this problem, but what are you going to do about it?’ ” Bernstein said. “I would argue that Democrats are going to have to have more in their toolbox than [raising the] minimum wage and universal pre-K.”