Susan Collins will vote against Education Department nominee Betsy DeVos, potentially imperiling her nomination.

The moderate Maine Republican will become one of the first GOP senators to vote against one of Trump's nominees. No Democrats are expected to vote for DeVos, a GOP megadonor and education advocate who has long pushed for charter schools and K-12 tuition vouchers using public funds.

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"This is not a decision I make lightly. I have a great deal of respect for Mrs. DeVos," Collins said on the floor on Wednesday afternoon. "I will not, can not vote to confirm her."

In an interview, Collins said that she will allow DeVos to advance to a final vote, before opposing her.

Sen. Lisa Murkowski of Alaska, another moderate Republican, is also opposed to DeVos. Both she and Collins voted for DeVos in committee on Tuesday, but reserved the right to oppose DeVos on the floor.

Both Republicans have expressed concerns about her support for voucher programs, which were emphasized in Donald Trump’s $20 billion school choice proposal rolled out on the campaign trail. DeVos has drawn bipartisan criticism for her views on education, though Republicans have been working to usher her through.

The margin for DeVos narrowed significantly after Sen. Joe Manchin (D-W.Va.) announced he will oppose DeVos. Manchin has been mostly supportive of Trump's nominees but said DeVos has shown a lack of understanding for public schools.

“Betsy DeVos has never attended or worked in a public school. The needs facing rural schools in West Virginia are unique and her lack of exposure to public education is very concerning for me," Manchin said. "Every child in West Virginia deserves a quality education and I do not believe that Betsy DeVos is qualified to serve in this role, which is so vital to the future of our state.”

She needs a simple majority to be confirmed; there are 52 Republicans. If there is a tie, Vice President Mike Pence will be the deciding vote.

Republican leaders had been planning to bring either DeVos or Attorney General nominee Jeff Sessions to the floor for a confirmation vote later this week. If Sessions is confirmed, Republicans may need to swear in a new senator from Alabama in order to be able to confirm DeVos.

Democrats were unhappy with DeVos from the start. But her lackluster performance at her confirmation hearing, in which she seemed confused by federal special education law and referenced a Wyoming school with a grizzly bear problem when asked about gun policy, made her a social media sensation and help galvanize the opposition.

Both Republican and Democratic senators say they've been flooded with thousands of phone calls, emails and letters related to DeVos’ nomination — mostly in opposition.