The Senate late Monday passed a resolution approving Leahy as president pro tempore. He would replace Hawaii Sen. Daniel Inouye, who died Monday.
The seven-term Leahy would be third in line to the presidency, behind the vice president and the speaker of the House.
Leahy also is in line to replace Inouye as chairman of the Senate Appropriations Committee. The 72-year-old Leahy currently is chairman of the Senate Judiciary Committee.