Note that by the 25th Amendment both houses would have to confirm. The (bare) Democratic majority Senate would be a challenge. The GOP majority House an even stronger hurdle. Though confirmation is a simple majority, not a supermajority (2/3 vote):

Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress.

<https://www.law.cornell.edu/constitution/amendmentxxv>

There's also the fact that in ascending to the Presidency, Harris would no longer be President of the Senate, which would lose her tie-breaking vote, AFAIU.