U.S. Sen. Cory Booker wasn't done with the fireworks Thursday.

Hours after saying he was willing to risk being ousted from the Senate by releasing confidential documents, the New Jersey Democrat openly questioned whether Republican President Donald Trump nominated Judge Brett Kavanaugh to the U.S. Supreme Court to protect himself amid criminal investigations.

"You've seen this president demanding loyalty," Booker told Kavanaugh during the afternoon session of Day 3 of the judge's confirmation hearing before the Senate Judiciary Committee in Washington, D.C.

"I'm just wondering what kind of loyalty is being required of you for this job," added Booker, who is considered a potential Democratic candidate to challenge Trump in the 2020 presidential election.

"It is understandable for people to suspect that there's something going on, that somehow this is rigged," the senator said.

Special Counsel Robert Mueller is investigating whether Russia interfered in the 2016 presidential election and whether Trump's campaign colluded with the country. Mueller is also examining whether Trump obstructed justice in the investigation.

Meanwhile, Trump's former personal lawyer, Michael Cohen, pleaded guilty to tax fraud and violating campaign finance rules, and said in court that Trump directed him to break those rules.

Trump has vehemently denied roles in any of those issues.

Booker referenced how Kavanaugh has written articles suggesting a president could not be indicted while in office.

Kavanaugh responded that the U.S. Justice Department has held that position for 45 years -- but also insisted he has "an open mind" as to whether that position is constitutional.

Booker also noted that Kavanaugh, who is currently a U.S. appeals court judge, was not on Trump's shortlist for possible Supreme Court picks until after the Mueller probe was launched.

And Booker pointed out that Kavanaugh, the former White House staff secretary under then-President George W. Bush, a Republican, has said he has "the greatest respect" for Bush and has spoken frequently about how the "character" of a president matters.

Booker then asked Kavanaugh if he also has "the greatest respect" for Trump.

Kavanaugh declined to answer, saying: "You don't hear sitting judges commenting on political (matters)."

"I appreciate the question," he added. "What I've said during this process is I need to stay away."

Booker accused Kavanaugh of "going mum as to the character of this president given all his lies, all his remarks that have been criticized on both sides of the aisle."

"Can you speak to that credible suspicion that people might have that the system is somehow rigged and the president is putting somebody up just to protect him from a criminal investigation?" the senator asked.

Kavanaugh replied that his "only loyalty is to the Constitution."

"I've made that clear. I'm an independent judge," he added.

Booker said he respects Kavanaugh for spending his "whole life in public service" but argued these are "unusual times" in America -- almost like "a fiction novel."

"The test for all of us is coming," the senator said. "The test for the Supreme Court is coming, as well. This is going to be a time, if we have a constitutional crisis, where the faith in this country will be tested, shaken again. And it's really important the Supreme Court be above suspicion."

He then asked Kavanaugh why he won't promise to recuse himself if anything involving Mueller investigation comes before the Supreme Court.

"If I committed to deciding a particular case ... all I'd be doing is demonstrating I don't have the independence of the judiciary that is necessary to be a good judge," Kavanaugh said.

Earlier Thursday, Booker made headlines by flaunting that he was breaking Senate rules by releasing confidential documents about Kavanaugh despite being warned he could be expelled from the Senate for doing so.

"I come from a long line, as all of us do as Americans, understand what that kind of civil disobedience is and I understand the consequences," Booker said.

"This is about the closest I'll have in my life to an 'I am Spartacus' moment,'" he added.

But Republicans said the documents had already been cleared for release and accused Booker of grandstanding and manufacturing the drama just to draw attention for his possible 2020 run.

Brent Johnson may be reached at bjohnson@njadvancemedia.com. Follow him on Twitter @johnsb01. Find NJ.com Politics on Facebook.