Who is Senate pro tempore Patrick Leahy and why is he presiding over Trump impeachment trial?

Supreme Court Chief Justice John Roberts presided over former President Trump's first impeachment trial a year ago for abuse of power and obstruction of Congress. 

But on Tuesday, Roberts will not be returning to the Senate to lead the second impeachment trial since Trump is out of office.

Who will preside?

That responsibility will fall to Sen. Patrick Leahy, D-Vt., who became the president pro tempore of the Senate in January when Democrats regained the majority.

LEAHY EXPECTED TO PRESIDE OVER TRUMP IMPEACHMENT TRIAL INSTEAD OF CHIEF JUSTICE ROBERTS

What is the president pro tempore?

The president pro tempore is typically the most senior U.S. senator of the party in power and Leahy indeed has served in the Senate longer than any other Democrat. The designation carries responsibilities as the third in line of succession to the presidency following the vice president and House Speaker.

"The president pro tempore has historically presided over Senate impeachment trials of non-presidents," Leahy said in a statement announcing his new role. "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."

On Jan. 26, Sen. Chuck Grassley, R-Iowa, swore in Leahy to preside over Trump's second impeachment trial. Grassley previously was Senate pro tempore when Republicans had the majority. 

Who is Leahy?

Leahy, 80, was first elected to the Senate in 1974 at the age of 34, making him the youngest senator in Vermont's history.  He previously served as president pro tempore when Democrats held power from 2012 to 2015.

"I consider holding the office of the president pro tempore and the responsibilities that come with it to be one of the highest honors and most serious responsibilities of my career," Leahy said. "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."

WHAT IS TRUMP BEING ACCUSED OF IN THE SENATE IMPEACHMENT TRIAL?

How has it gone for Leahy?

The impeachment trial got off to a shaky shart for Leahy. Shortly after being sworn in to preside over the trial, Leahy was admitted to George Washington University Hospital that evening for observation "out of an abundance of caution," his spokesperson said on Jan. 26.

Leahy was sent home after getting his test results back and has since been back at work. 

Leahy is able to still vote with the rest of his Senate colleagues on whether Trump should be convicted. Trump is facing one article of impeachment for "incitement of insurrection" related to the Jan. 6 Capitol riot. 

Sen. John Cornyn, R-Texas, suggested Leahy's double roles could be a conflict of interest.

"How does a Senator preside, like a judge, and serve as juror too?" Cornyn tweeted on Jan. 25.

One of Leahy's first jobs as the presiding officer was to sign the official summons to the trial for former President Trump. 

When the Senate convenes on Tuesday afternoon as a court of impeachment, Leahy's public duties will begin in earnest as he takes the center stage seat in the presiding officer's chair. 

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It's unlikely that Trump could be convicted at a Senate trial, as 17 Republicans would need to join all 50 Democrats to reach the two-thirds threshold. But if Trump is convicted, he could be barred from holding office in the future by a simple majority on a subsequent vote. 

Fox News' Tyler Olson contributed to this report.