Trump's impeachment trial is scheduled to begin the week of February 8, US Senate Majority Leader Chuck Schumer said earlier.
"When I preside over the impeachment trial of former President Donald Trump, I will not waver from my constitutional and sworn obligations to administer the trial with fairness, in accordance with the Constitution and the laws," Leahy said.
Leahy has served as a Senator from the US state of Vermont for 46 years, first elected in 1975. As the longest-serving US Senator, he serves as president pro tempore of the Senate, chairing the upper chamber of Congress when the US vice president, currently Kamala Harris, is not available.
"The president pro tempore has historically presided over Senate impeachment trials of non-presidents. When presiding over an impeachment trial, the president pro tempore takes an additional special oath to do impartial justice according to the Constitution and the laws. It is an oath that I take extraordinarily seriously," Leahy noted.
A president pro tempore of the Senate is third in line to succeed a US president, currently Joe Biden, after the US vice president, and the Speaker of the House, currently Nancy Pelosi.
