WASHINGTON — After years of procrastination, the White House and Congress have suddenly boosted a long-term plan to improve the nation's roads, bridges and transit systems to the top of the political agenda.

This week, the House and Senate are set to take up vastly different bills providing a blueprint for shoring up the nation's aging transportation system. Lawmakers are driven in part by a desire to show voters a major accomplishment in an election year. They are pitching the bills as jobs generators, although it may be more accurate to say they preserve jobs that might otherwise be lost if Congress doesn't find a way to keep highway and transit programs solvent.

President Barack Obama chimed in Monday with his own plan to spend nearly half a trillion dollars over six years on transportation infrastructure. And on Tuesday he threatened to veto the House bill, saying the Republican measure doesn't spend enough to strengthen the nation's transportation system and weakens environmental and labor standards.

But since the president's plan is much grander than anything Congress is likely to go along with, the administration has swung its weight behind the Senate bill, a plan that more modestly proposes to spend $109 billion over less than two years.

House Republicans have proposed spending about $260 billion over nearly five years, but the bill is drawing fire from so many quarters that the ability of Speaker John Boehner, R-Ohio, to muscle the measure through to passage is in doubt. To build support with tight-fisted tea partiers for such a large spending bill, GOP leaders added sweeteners designed to appeal to conservatives, such as expanded offshore oil and gas drilling, approval of the controversial Keystone pipeline and a requirement that federal employees pay more toward their pensions.

The bill's treatment of mass transit programs has riled urban lawmakers, who may wind up voting against the bill. It eliminates the guarantee of a portion of federal gasoline and diesel tax revenues for transit, leaving programs vulnerable to future budget cuts.

Lawmakers could resolve much of the funding woes by increasing the 18.4 cent-a-gallon gas tax and the 24.4 cent-a-gallon diesel tax, but that's politically unpalatable.

Indecision about how to shore up the Highway Trust Fund has long stymied efforts to pass a transportation plan. The last long-term plan expired in 2009. Congress has kept programs going through a series of eight short-term extensions; the current one expires March 31.