WASHINGTON — A crucial first Senate vote on President Barack Obama's health care overhaul in a rare Saturday night session looms as a test of Democratic unity and the president's prestige.

Democratic leaders are optimistic of success, but they need every Democrat and both independents to vote "yes," and two moderates remained uncommitted ahead of the roll call, which is expected around 8 p.m.

The vote will determine whether debate can go forward on Majority Leader Harry Reid's 2,074-page bill to dramatically remake the U.S. health care system over the next decade.

The two holdouts are Sens. Blanche Lincoln of Arkansas and Mary Landrieu of Louisiana. A third centrist, Ben Nelson of Nebraska, announced Friday he would be supporting his party on the test vote, while cautioning that it didn't mean he would be with them on the final vote.