By ERICA WERNER Associated Press Writer

WASHINGTON (AP) - 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.

Most everyone would be required to purchase insurance under Reid's legislation, and billions in new taxes would be levied on insurers and high-income Americans to help extend coverage to 30 million uninsured. Insurance companies would no longer be allowed to deny coverage to people with medical conditions or drop coverage when someone gets sick.

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

"It is not for or against the new Senate health care bill," Nelson said. "It is only to begin debate and an opportunity to make improvements. If you don't like a bill, why block your own opportunity to amend it?"

If that same reasoning holds with Lincoln and Landrieu, Reid, D-Nev., will have the 60 votes he needs to prevail in the 100-seat Senate. The 40 Republicans are unanimously opposed.

Landrieu has made comments suggesting she'll support the move to debate, but Lincoln, who faces a difficult re-election next year, carefully avoided taking any public position Friday.

Republicans took to the Senate floor to slam the bill as a government takeover that would increase taxes and lead to rationing of health care.

"Washington will tell you what is good enough coverage," said Sen. Mike Enzi, R-Wyo. "These insurance changes will increase costs for millions of Americans."

Democrats said their legislation could make historic and necessary improvements in the country's social safety net.