UPDATE 6-Obama kicks off campaign with infrastructure plan

* Infrastructure overhaul would create jobs from 2011

* Obama eyes oil, gas firm tax breaks to pay for it

* Republicans skeptical about "cobbled-together" stimulus

* Analysts say plan would not help economy immediately (Recasts, adds details, reaction)

By Steve Holland

MILWAUKEE, Sept 6 (Reuters) - President Barack Obama, scrambling to jump-start job creation in a sluggish U.S. economy, proposed a six-year plan on Monday to rebuild aging roads, railways and runways with an initial $50 billion investment.

"We are going to rebuild 150,000 miles (240,000 km) of our roads -- that's enough to circle the world six times. ... We're going to lay and maintain 4,000 miles (6,400 km) of our railways -- enough to stretch coast-to-coast," Obama told a labor rally in Milwaukee where several thousand supporters cheered his every line.

The infrastructure plan, one of several initiatives Obama is due to unveil this week, was immediately rejected by Republicans, who many analysts predict could win control of the House of Representatives in Nov. 2 congressional elections.

With fellow Democrats facing punishment from recession-weary voters in November, Obama is under pressure to do more to create jobs and bring down the stubbornly high 9.6 percent unemployment rate, even as economists agree he has few good options left.