President Barack Obama proposed spending $50 billion next year on highways, airport and railway construction, the latest attempt to boost the economy, although the White House message on the speed of the impact was mixed and the plan ran into immediate opposition from Republican lawmakers.

At a Labor Day rally in Milwaukee, Mr. Obama said the initaitive would be part of a larger effort to fix 150,000 miles of roads, lay or rebuild 4,000 miles of railroad track and refurbish some 150 miles of airport runways. The proposal...