Senate Majority Leader Chuck Schumer told Democrats on Wednesday that he won't nominate Sen. Dianne Feinstein, 89, to be the Senate president pro tempore in the next Congress amid ongoing of speculation about her mental acuity.

A source familiar with the caucus discussion told Fox News on Tuesday that Schumer, D-N.Y., will instead pick Sen. Patty Murray, D-Wash., for the job.

The Senate president pro tempore is a position laid out by the Constitution to chair the Senate when the vice president is not present. It's a largely powerless office as Senate party leaders control legislative traffic on the floor.

The Presidential Succession Act, however, makes the president pro tempore third in the line of succession if the president of the United States dies, following the vice president and House speaker.

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Dianne Feinstein in the Capitol

Sen. Dianne Feinstein, D-Calif., won't be the next Senate president pro tem. (Kevin Dietsch/Getty Images)

Traditionally, the president pro tempore job goes to the most senior senator in the majority party. Currently, that's Sen. Patrick Leahy, D-Vt., who is retiring, which leaves Feinstein the most senior Democrat in the chamber.

But several media reports cite declines in Feinstein's mental acuity, including her memory, which could make putting her in a position of such responsibility untenable. Schumer will avoid that conversation by nominating Murray, the second-most senior Democratic senator, for the job.

"I am honored to have earned the confidence of Leader Schumer and my colleagues to serve as President Pro Tempore. It’s not lost on me the significance of what it would mean to be the first woman to serve in this role," Murray said in a statement. "I have a great deal of respect for the Senate and the good I am able to accomplish for families in Washington state as a voice and vote for them."

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The Senate will likely name Murray its president pro tempore at the beginning of the new Congress.

If Republicans had won the Senate majority, Sen. Chuck Grassley, R-Iowa, also 89 years old, would have likely been the next president pro tempore. He held the office the last time Republicans were in the majority.

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But Democrats held onto a slim Senate majority in the midterms. The Georgia Senate runoff set for Dec. 6 will decide if it's a 51-49 majority or another 50-50 Senate in which Vice President Kamala Harris can break ties.

Fox News' Chad Pergram contributed to this report.