By Lisa Lambert

WASHINGTON (Reuters) - Billionaire philanthropist Betsy DeVos, already known as one of the most controversial nominees for education secretary in U.S. history, now risks a rare congressional rejection.

The deeply divided U.S. Senate Health, Education, Labor and Pensions Committee on Tuesday agreed to send her nomination to the full chamber for a vote, the final step in the confirmation process.

But the committee's executive session showed DeVos faces choppy waters ahead for a post for which there is typically little congressional debate or public attention.

The chairman, Republican Lamar Alexander, acted as tie-breaker after all 11 Republicans voted for Republican President Donald Trump's pick and all 11 Democrats voted against.

Two Republicans - Maine's Susan Collins and Alaska's Lisa Murkowski - expressed grave misgivings about the charter school advocate's limited experience with public schools. They said they voted yes only so the entire Senate can debate whether DeVos is the right fit. Murkowski said she may not support DeVos in the Senate vote.

Democrats said the nomination was rushed, with DeVos providing answers they described as vague and noncommittal to hundreds of written questions only 24 hours before the vote.

After many of Trump's nominees sailed through their confirmations in recent weeks, the names currently before the Senate were meeting resistance.

The Senate's 48 Democrats only need to pull three Republicans to their side to reject DeVos' nomination. Just nine nominees have ever been turned down by the Senate.

Public opposition to DeVos is unusually fierce, said Mary Kusler, lobbyist for the National Education Association, a large union that said constituents sent 1 million emails and placed 40,000 calls to senators decrying the nomination.

It was the biggest public outcry over an education secretary that Kusler has seen in her career of more than 20 years, she added.

NEA and the American Federation of Teachers, another major union, say they will keep up their anti-DeVos fight during the Senate vote and expect voters will also maintain pressure.

Public opinion of DeVos wobbled after her Jan. 17 hearing, where she suggested allowing guns at rural schools to fend off bear attacks.

At two rallies on Tuesday in Chicago, several hundred protesters gathered to demonstrate against DeVos' nomination.

“I have one child in school, and my son will be (in school) in a little over a year,” said Chicago resident Mara Tierney, who attended a rally with her 3-year-old boy. “I want to do what I can to ensure DeVos is not confirmed.”

Old-guard Republicans like former first lady Barbara Bush defended DeVos as a lifelong champion of low-income children, literacy and giving parents more choice in education.

Democrats, meanwhile, blasted the nominee for supporting privatization in ways they said hurt poor students, not understanding basic education laws and issues, and courting potential conflicts of interest through her investments.

(Reporting by Lisa Lambert; additional reporting by Bob Chiarito in Chicago.; editing by Chizu Nomiyama and Jonathan Oatis)