Story highlights DeVos is still expected to be confirmed as secretary of education

Vice President Mike Pence may be called upon to cast a tie-breaking vote in the Senate

Washington (CNN) Republican Sens. Susan Collins of Maine and Sen. Lisa Murkowski of Alaska both said Wednesday they would not vote to support Betsy DeVos, President Donald Trump's choice for education secretary. The two women are the first sitting Republican senators to publicly say they wouldn't back one of Trump's Cabinet nominees.

"I simply cannot support her confirmation," Collins said on the Senate floor.

The stunning decision from a pair of veteran Republican lawmakers puts DeVos in a tenuous position as her final vote before the Senate nears, but top Republican aides are confident she will be confirmed.

If Republicans can maintain 50 votes in support of DeVos, then Vice President Mike Pence would become the tiebreaking vote to confirm her.

In order to help make this happen, Senate Majority Leader Mitch McConnell decided to move up the vote on DeVos ahead of the planned vote on Sen. Jeff Sessions for attorney general. Should Sessions leave the Senate before the DeVos vote, there would only be 49 Republicans committed to voting for her.

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