University of North Carolina law professor Michael Gerhardt, a witness called by Democrats, told the committee the Constitution's founders designed an impeachment inquiry to have several safeguards -- and expressed concern that they are not be adhered to in this case.

The first safeguard, he said, is credible evidence of the commission of treason, bribery and other high crimes and misdemeanors.

"There is not, at least not that I've yet heard, such credible evidence," Gerhardt said.

"Let me give you an example of what I fear is similar to the current proceedings: Hunter Biden is arrested for speeding in a car owned by his father, and the police go after the father. I don't think that's how the law should work. I don't think that's how impeachment should work."

He also stressed the importance of judicial review, telling the panel the U.S. Supreme Court previously decided that the House Oversight Committee has to conduct an investigation for a legitimate purpose.

"A fishing expedition is not a legitimate purpose," he said.

Jonathan Turley, a George Washington University law professor and Republican witness, said in his opening statement he didn't believe impeachment articles against President Biden were warranted at this time.

"I want to emphasize what it is that we're here today for," he said. "This is a question of an impeachment inquiry. It is not a vote on articles of impeachment. In fact, I do not believe that the current evidence would support articles of impeachment. That is something that inquiry has to establish."

Turley, also a Fox News contributor, went on to say he supported the impeachment inquiry because of the GOP allegations Biden spoke falsely about foreign business deals and possibly benefitted from his family's business dealings -- but emphasized they're "merely allegations and they should not become presumptions of impeachable conduct."

Democratic Rep. Jamie Raskin, the committee's ranking member, didn't mince words as he tore into Republicans for moving ahead with an impeachment inquiry when Congress is barreling toward a government shutdown.

"We're 62 hours away from shutting down the government of the United States of America and Republicans are launching an impeachment drive based on a long debunked and discredited lie," Raskin said.

"No foreign enemy's ever been able to shut down the government United States but now MAGA Republicans are about to do just that. But they don't want to cut off public services to the people and deny paychecks to more than a million service members without first launching the impeachment drive, even when they don't have a shred of evidence against President Biden for an impeachable offense."

Raskin went on to point to poster boards filled with recent quotes from Republicans on the spending showdown, including comments like "clown show" and "new low."

Raskin also criticized Republicans for launching an impeachment inquiry without a full House vote, and said they sit "empty handed."

As staffers displayed a poster with former President Donald Trump's posts on his social media platform calling for a government shutdown and Biden's impeachment behind him, Raskin slammed Republicans actions.

"We've moved from a Trump-ordered Republican shutdown to a Trump-ordered impeachment hearing. Back in the reality-based world, the majority sits completely empty handed with no evidence of any presidential wrongdoing, no smoking gun, no gun, no smoke," Raskin said. "If the Republicans had a smoking gun or even a dripping water pistol, they would be presenting it today. But they've got nothing," he said.

Comer, in his opening statement, said President Biden lied when he said he never spoke with his family about their business dealings and continued to claim, without direct evidence, that Biden was improperly influenced by his family's business affairs.

"At least 10 times Joe Biden lied to the American people that he never spoke to his family about their business dealings," Comer said. "He lied by telling the American people that there was an absolute wall between his official government duties and his personal life."

Biden has previously unequivocally claimed he "never discussed" business with his son but Republicans have pointed to testimony from Hunter Biden's former business partner Devon Archer, who testified that Joe Biden attended at least two dinners with Hunter Biden's foreign business associates, and frequently spoke with his son over the phone while in the presence of foreign business associates.

But Archer also testified he never heard business being discussed during those interactions. "The conversation is generally about the weather and what it's like in Norway or Paris or wherever he may be. … But it was very casual conversations, not about cap tables or financials or anything like that," Archer testified.

"The American people demand accountability for this culture of corruption," Comer continued in his opening statement. "They demand to know how these schemes have compromised President Biden and threaten our national security. They demand safeguards to be put in place to prevent public officials from selling access to their public office for private gain."

The committee is expected to reexamine the findings of months of GOP-led investigations and offer an explanation as to why an inquiry is warranted.

"This week, the House Oversight Committee will present evidence uncovered to date and hear from legal and financial experts about crimes the Bidens may have committed as they brought in millions at the expense of U.S. interests," chairman James Comer, R-Ky., said in a statement.

The committee will hear from four witnesses, three of whom were called by Republicans to provide testimony.

The panel's 46 members (plus other lawmakers) will be allowed to question the witnesses in a hearing that could stretch on for more than six hours.

Read more about what to expect from the hearing here.