MILWAUKEE  President Obama, looking to stimulate a sluggish economy and create jobs, called Monday for Congress to approve major upgrades to the nation’s roads, rail lines and runways  part of a six-year plan that would cost tens of billions of dollars and create a government-run bank to finance innovative transportation projects.

With Democrats facing an increasingly bleak midterm election season, Mr. Obama used a speech at a union gathering on Labor Day, the traditional start of the campaign season, to outline his plan. It calls for a quick infusion of $50 billion in government spending that White House officials said could spur job growth as early as next year  if Congress approves.

That is a big if. Though transportation bills usually win bipartisan support, hasty passage of Mr. Obama’s plan seems unlikely, given that Congress has only a few weeks of work left before lawmakers return to their districts to campaign and that Republicans are showing little interest in giving Democrats any pre-election victories.

Central to the plan is the president’s call for an “infrastructure bank,” which would be run by the government but would pool tax dollars with private investment, the White House says. Mr. Obama embraced the idea as a senator; with unemployment still high despite an array of government efforts, the concept has lately been gaining traction in policy circles and on Capitol Hill.