A former top Democrat has emerged as the frontrunner to be FBI director

Former Connecticut senator and Democratic vice-presidential nominee Joe Lieberman has emerged as the favorite to be named the next FBI director.

The position is expected to be filled either Thursday or Friday. Trump replied affirmatively in the Oval Office on Thursday when a reporter asked if Lieberman was a finalist.

Lieberman's front-running status, first reported by Politico, came following the former senator's Wednesday meeting with Trump, when a person familiar with the meeting told Politico that Trump "bonded with Lieberman."

The vacancy opened up after Trump's shocking ouster of FBI Director James Comey, who was overseeing a counterintelligence investigation into potential collusion between the Trump campaign and Russian officials in the 2016 presidential election. On Wednesday, Deputy Attorney General Rod Rosenstein appointed a special counsel, former FBI Director Robert Mueller, to oversee that investigation.

If Lieberman was selected to be FBI director, it would represent a stark change for the bureau. Of the seven past FBI directors, none were former politicians.

Lieberman served in the Senate from 1989 through 2013. In 2000, he was Democratic presidential nominee Al Gore's running mate. But in 2006, he left the Democratic Party and became an independent. Politico reported that Democratic Sen. Dick Durbin and others were critical of a potential Lieberman nomination, according to someone familiar with the discussion.

Since leaving the Senate, Lieberman joined the law firm Kasowitz, Benson, Torres, and Friedman, where he serves as senior counsel. The firm has represented Trump for years, which could present a sticking point for both Democrats and Republicans during a potential confirmation. But the appointment of Mueller, combined with Lieberman's longstanding friendships with members of both parties, could quash some of those foreseeable complaints.

One Democratic senator, Claire McCaskill of Missouri, brushed back on the idea of Lieberman as FBI director on Thursday, saying the person nominated should be someone who's "never" run for office.

Lieberman, one of four people to meet with Trump to discuss the open FBI position on Wednesday, introduced Trump's pick to lead the Department of Education, Betsy DeVos, during her Senate confirmation hearing in January. Lieberman serves on the board of a DeVos-founded education organization.

The former Connecticut senator would be the oldest FBI director at the start of his tenure at the age of 75. FBI directors are appointed for 10-year terms by the president.

Lieberman has weighed in on some points of Trump's agenda since his election. In January, he said he wasn't surprised that Trump delivered on his campaign promise to restrict the travel of people from some terrorism hotspots to the US.

"Like everything else, this was to be expected, President Trump is keeping his promise," Lieberman, the former chairman of the Senate Homeland Security Committee, told Business Insider in an interview. "The question is how the administrative agencies, in this case immigration, carry out his order."

Lieberman also addressed Trump's proposal for a wall along the US-Mexico border, saying he didn't believe Trump could get Mexico to pay for it "voluntarily under any means that I can see." Lieberman also addressed Trump's travel ban, which was later halted by the courts, saying that "like everything else, this was to be expected, President Trump is keeping his promise. The question is how the administrative agencies, in this case immigration, carry out his order."