President Donald Trump's choice of billionaire Betsy DeVos to be education secretary was confirmed by the U.S. Senate on Tuesday, but only after Vice President Mike Pence was called in to break a tie that threatened to defeat her.

The tie-breaking vote, which Senate officials said was unprecedented to confirm a president's Cabinet nominee, came after two Republicans joined with 46 Democrats and two independents in opposition to DeVos. Critics have called her unprepared to lead the Department of Education.

DeVos is married to the heir and former CEO of Amway, which sells household and personal care items. She is also the daughter of the founders of Prince Corporation, a Michigan car parts supplier and sister of Erik Prince, the founder of the security company formerly known as Blackwater USA, now called Academi.

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Under the U.S. Constitution, the vice president also serves as president of the Senate. As Senate president, he has the power to cast votes only when there are ties on nominations or legislation.

Once the Senate votes to confirm or reject the DeVos nomination, it is expected to move on to debating Trump's choices of Senator Jeff Sessions to be attorney general, Representative Tom Price to be secretary of Health and Human Services and ex-banker Steve Mnuchin to be Treasury secretary.

All three face opposition from Senate Democrats.