WASHINGTON — President Obama on Monday called for a $50-billion surge in spending on the nation’s roads, runways and railroads, his latest effort to respond to the stubbornly sluggish economy in a political climate turning against his party.

Speaking at a union-organized rally in Milwaukee, the president said his proposal would put construction workers back to work and rebuild deteriorating infrastructure.

“”It’s a plan that says, even in the aftermath of the worst recession in our lifetimes, America can still shape our own destiny, we can still move this country forward, we can still leave our children something better — something that lasts,”” the president said, in a campaign-style speech that sought to make the case for his economic policies.

The Labor Day speech came as Democratic candidates were launching the final leg of their campaigns and, in many cases, facing a harsh assessment from voters frustrated by the pace of economic recovery. Polls show voters feel uneasy about the economy, unhappy with the rising deficits and willing to give Republican policies a try.

The White House will use this week to show it is reacting to that climate. The infrastructure plan was slated to be the first in a series of proposals unveiled by the president, although experts have cast doubt on whether the administration can do much to improve the job market before November.

Senior administration officials on Monday declined to make predictions about how many job could be created by the infrastructure plan. They said only that they expected results next year.

They resisted describing the $50-billion proposal as a stimulus measure, instead casting it as part of a six-year transportation plan to be put before Congress in the fall. The bill would be “”front-loaded”” to spend money quickly in hopes of creating jobs in the short term, they said.

The officials declined to put a dollar figure on the total amount of new spending in the transportation proposal, saying only that the $50 billion would be a significant percentage of the overall increase. The White House hoped to cover the costs by closing a series of tax breaks for oil and gas companies, they said.

“”This is a plan that will be fully paid for and will not add to the deficit over time,”” Obama said. “”We’re going to work with Congress to see to that.””

Still, it is unclear whether Congress will have the appetite for another major spending bill. Congressional Democrats have spent much of this campaign season defending themselves against Republican charges of wasteful and unnecessary spending that has ballooned the deficit with few results. An $814-billion stimulus package passed last year has not proven to be popular, particularly with independent and fiscally conservative voters.

On Monday, Republican Whip Eric Cantor quickly sought to label Obama’s plan as more of the same.

“”Today the president will use the Labor Day holiday as the launching pad for yet another government stimulus effort, another play called from the same failed Keynesian playbook,”” Cantor said in a statement released before Obama announced his plan in Milwaukee. “”More government stimulus does nothing to end this cloud of uncertainty.””

The $50-billion plan outlined Monday amounted to roughly half of what was allocated to infrastructure in last year’s stimulus package. Officials said the proposal project would build on the work already begun.

The initial spending would be used to build or repair 150,000 miles of road, 4,000 miles of railways and 150 miles of airport runways, as well as to support a modernization of the air-traffic-control system.

The president also planned to call for the creation of an infrastructure bank that would leverage funding for projects that reached a regional or national scale. Over the next several years, the plan would continue funding for high-speed rail, consolidating more than 100 small programs, and prioritize investments in projects that lower greenhouse-gas admissions and reduce oil consumption.

The plan was proposed as the unemployment rate stood at 9.6 percent and the economy continued to lose jobs, according to a Labor Department report issued last week.