By David Morgan

WASHINGTON, Jan 16 (Reuters) - At least 22 U.S. governors, including four Republicans, support an expansion of Medicaid under President Barack Obama's healthcare reform law, while others are expected to decide in the coming weeks, experts said on Wednesday.

An analysis published by the New England Journal of Medicine said the headcount, which includes 13 Republican governors who staunchly oppose Medicaid expansion, portends an uneven start for "Obamacare" when its most sweeping reform provisions begin on Jan. 1, 2014.

"No clear consensus has emerged," said the authors of the article, Harvard physicians Benjamin Sommers and Arnold Epstein. "It now appears that the (law's) 2014 coverage expansion will have large unintended gaps, as low-income adults in at least a dozen states remain ineligible for any kind of public subsidy for health insurance."

The analysis provides a snapshot of the Medicaid acceptance among states as most governors and state lawmakers prepare for their last legislative session before the law takes effect. A governor's position does not necessarily indicate whether a state will embrace the expansion because many require action by legislatures.

The Patient Protection and Affordable Care Act, the signature domestic achievement of Obama's first term, calls for expanding the Medicaid program for the poor to include Americans living near the poverty line. In many states, current Medicaid benefits are limited mainly to children and pregnant women.

The expansion is intended to bring 16 million uninsured people into the $2.8 trillion U.S. healthcare system. But many states, which would pay less than 10 percent of the cost under the law, have balked at the potential budget impact or rejected the prospect of participating in a new government program.

States that oppose the expansion can continue to operate their current Medicaid programs with federal matching funds.

Since Obama's reelection, eight governors have chosen to support the Medicaid expansion, including the Republican governors of Arizona, Nevada, New Mexico and North Dakota. Five Republicans have chosen to oppose it.

Another 15 governors -- 13 Republicans and two Democrats -- remain undecided. But experts say many of them are likely to reach decisions in coming weeks as they propose new budgets for the fiscal year beginning July 1.

"We're looking to those current budgets, particularly for state that have been undecided, to see if they might have a more definitive position," said Robin Rudowitz, a Medicaid expert with the nonpartisan Kaiser Family Foundation, which tracks healthcare issues.

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