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 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 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. Polls show voters feel uneasy about the economy, unhappy with the rising deficits and willing to give Republican policies a try.

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.

The $50 billion plan outlined Monday amounted to roughly half of what was allocated to infrastructure in last year’s stimulus package.

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.