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

Rep. Dennis Kucinich, D-Ohio, long a supporter of Medicare-for-all, voted against the House Democratic bill in November because it did not go far enough in creating a robust government-run plan to compete with private insurance. But Kucinich said today the bill coming before the House represents the best chance to expand coverage to the uninsured, even if it does not include a public plan.

At a Capitol Hill news conference, Kucinich said his decision was a combination of pragmatism and concern about the impact that defeat of the health care bill would have on Obama's presidency.

"You do have to be very careful that the potential of President Obama's presidency not be destroyed by this debate," Kucinich said. "Even though I have many differences with him on policy, there's something much bigger at stake here for America."

At stake is a bill that would cover some 30 million uninsured people, 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 cost of medical care nationwide. The comprehensive legislation could affect nearly every American, from those undergoing annual checkups to people facing major surgery.