New Jersey Sen. Cory Booker (D.) suggested blocking President Donald Trump's upcoming Supreme Court nomination on Thursday, saying it should be halted because the president is under "criminal investigation."

In the wake of Supreme Court Justice Anthony Kennedy's announced retirement, MSNBC's Ari Melber asked Booker how he can take a position on Trump's future nominee without knowing who it is.

"This president is the subject of a criminal investigation," Booker said. "Here you have a president that's making a whole lot of loyalty tests and other things from people."

Booker also said the future justice could end up presiding over cases relevant to the investigation.

"Can a president pardon himself? Can a president be criminally indicted? Can a president dismiss the person that's investigating them?" Booker hypothesized as issues that could all come before the Supreme Court.

He repeated that Trump has a history of loyalty tests.

"Should that person be able to right now, while this criminal investigation's going on, be allowed to make an appointment? I say no," Booker said.

Booker testified as a character witness on behalf of his colleague Sen. Bob Menendez (D., N.J.), when he was on trial for public corruption for accepting bribes from his long-time friend Dr. Salomon Melgen, in exchange for political favors. The trial ended with a hung jury and the Department of Justice decided not to refile charges.

Melber asked if Booker really meant that Trump shouldn't be able to fill the nomination with the Mueller probe still going on.

"I am saying the Senate should be acting in a responsible fashion," Booker said, warning that Democrats and Republicans foresee the potential for a constitutional crisis.

Booker also warned that Trump could "put people in place that could come back and protect him."

"Do you think this president would look to fill the vacancy to try to protect himself against the issues you mentioned?" Melber asked.

"Yeah," Booker said. "I mean, here we have a president who said if he knew that Jeff Sessions would properly act to recuse himself, he wouldn't have appointed Jeff Sessions."

Booker also said Trump may look to pardon himself.

"Taking a pause now, waiting until the outcome of the criminal investigation of which President Trump is the subject of, it seems like the responsible thing to do to me," Booker said.

UPDATE: Friday, 8:56 A.M.: The article initially stated Booker "inaccurately" called Trump the subject of a criminal investigation. While Trump is not a "criminal target" of the probe according to Robert Mueller's team, he is a "subject" of the investigation. This has been corrected.