WASHINGTON (Reuters) - Public support for President Barack Obama's handling of healthcare reform, the pillar of his legislative agenda, has fallen below 50 percent for the first time, a Washington Post-ABC News poll released on Monday said.

Obama and his Democratic allies in Congress have run into stiff opposition this month as they try to pass legislation to restructure the $2.5 trillion U.S. healthcare industry through the creation of a government-run health insurance program.

Republicans and some fiscally conservative Democrats argue the plan, with an estimated cost of more than $1 trillion, could hurt small businesses, add to budget deficits and reduce the quality of medical care for many Americans.

Those concerns may be having an impact on the public, according to the poll, which showed 49 percent of respondents approving of Obama's stand on the issue compared to 57 percent in April.

Those saying they disapproved rose to 44 percent from 29 percent during the same period.

Obama and the White House have gone on the offensive to drum up support for the plan, which would compete with private insurers, provide cover to many of the 46 million uninsured and try to stem runaway medical costs.

With time running out to pass a bill in Congress this year, the battle is shaping up as a major test of Obama's presidency.

Delaying legislation until 2010, a congressional election year, could give Republicans and critics in the healthcare sector more time to galvanize opposition to the plan.

But Obama remains more trusted than Republicans in Congress to do a better job on healthcare reform, the poll showed, with 54 percent of respondents putting their faith in the U.S. leader versus 34 percent in favor of Republican lawmakers.

His overall approval rating also remains high at 59 percent despite some slippage in approval ratings for his handling of the economy, the federal budget deficit and other leading domestic issues, according to the poll.

It surveyed 1,001 adults randomly by telephone between July 15-18, 2009. The results from the full survey have a margin of error of plus/minus three percentage points.

(Writing by Paul Simao; Editing by Louise Ireland)