WASHINGTON - U.S. President Barack Obama's much-challenged health care overhaul gained traction Wednesday as a liberal lawmaker became the first to switch his opposition and Roman Catholic nuns declared their support in an unusual public break with their bishops.

Democratic leaders are hoping to vote on the bill this weekend, but it is not clear if they have enough votes to pass it. The bill is Obama's top domestic priority and the health care issue could help determine whether Democrats retain control of Congress in the November election.

On Wednesday, Obama's plan won the support of Dennis Kucinich, a former candidate for the Democratic presidential nomination.

Kucinich had voted against an earlier version of the health bill in November because it did not go far enough in creating a robust public plan to compete with private insurance. But Kucinich said Wednesday that the bill represents the best chance to expand coverage to the uninsured, even if it does not include a public plan.

At a news conference, Kucinich said his decision was a combination of pragmatism and concern about a defeat of the bill that would weaken Obama.

"You do have to be very careful that the potential of President Obama's presidency not be destroyed by this debate," Kucinich said.

Obama's health care plan would extend coverage to some 30 million uninsured Americans, end insurance practices such as denying coverage to those with a pre-existing conditions, require almost all Americans to get coverage by law and try to slow the rising cost of medical care.

House Democrats had passed the earlier version of the health bill with only a slim majority. Dozens of Democrats opposed it, and only one Republican supported it. Since then, Democratic leaders have lost votes because of several vacancies and a reversal by the one Republican. In addition, some Democrats who had supported the bill are now likely to oppose it because of concerns about abortion language.

That means Democratic leaders' must win over party members who, like Kucinich, had voted against the earlier bill. Unlike Kucinich, most of those lawmakers are from conservative districts where voters fear the bill will drive up taxes, widen the deficit and damage the quality of health care.

Meanwhile, in an unusual public disagreement that will reverberate among America's 70 million Roman Catholics, leaders of religious orders representing 59,000 nuns sent lawmakers a letter urging they pass the Senate's version of the health care bill. The measure contains abortion funding restrictions that the bishops say do not go far enough.

"Despite false claims to the contrary, the Senate bill will not provide taxpayer funding for elective abortions," said the letter signed by 60 leaders of women's religious orders. "It will uphold longstanding conscience protections and it will make historic new investments ... in support of pregnant women. This is the real pro-life stance, and we as Catholics are all for it."

The U.S. Conference of Catholic Bishops and the National Right to Life Committee have denounced the bill as a backdoor subsidy for abortion. But the nuns and the Catholic Health Association -- representing some 600 hospitals -- say restrictions in the Senate bill would still prevent taxpayer funding for abortion, although the legal mechanism for doing so is different from what the bishops prefer.