WASHINGTON, Dec 8 (Reuters) - The U.S. Senate was expected to vote on Wednesday on a Republican measure to up-end Democratic President Joe Biden"s COVID-19 vaccine mandate for private business, a move that appeared to have enough support from Democrats to succeed. The legislation would overturn administration rules ordering businesses with 100 workers or more to require vaccinations or coronavirus testing for millions of employees. It was likely to pass the Senate but would face strong headwinds in the House of Representatives and a threatened presidential veto. The initiative underscores the intense political opposition facing administration efforts to combat COVID-19 at a time when health officials are trying to contain the highly contagious Omicron variant. Asimilar Republican effort failed in the Senate last week after risking a government shutdown. Republicans said they have been inundated with calls from businesses with up to 500 workers that are concerned about having to fire employees who oppose COVID-19 vaccines and testing. "It"s got Main Street America scared," said Republican Senator Mike Braun, who is spearheading the legislative drive. The White House said in a statement on Tuesday that employers would face no burden from the mandate because the vast majority of American adults are fully vaccinated and noted that the rule exempts small businesses. Biden"s private business mandate has already been shut down by a federal appeals court. AU.S. judge in Georgia also blocked a Biden vaccine mandate aimed at federal contractors on Tuesday. The measure is not subject to Senate rules that require 60 of its 100 members to agree on most legislation, meaning it could pass with just a 51-vote simple majority. Two Senate Democrats -- Joe Manchin and Jon Tester -- were expected to join 50 Republicans in voting for the bill. "We are in the middle of a public health crisis. Everyone sees the damage it causes," Senate Majority Leader Chuck Schumer, the top Democrat, said in a speech. "The way to solve this is to be driven by science." The United States has the world"s highest daily average of new coronavirus infections. The virus has infected 49.5 million Americans and killed more than 794,000, according to the Reuters COVID-19 Tracker. But Republicans touted the measure"s bipartisan support as a clear message of public concern about the mandate"s potential impact on workers, businesses and the economy. The chamber"s top Republican, Mitch McConnell, branded the Biden mandate as an "illegal" and "absurd" government effort to micromanage the lives of private citizens. "This is not how things work in our country, period," McConnell said.