House Judiciary Committee Chairman Jerrold Nadler Thursday accused President Trump of putting his own personal interests above national security and American democracy and charged that Trump is the only president in history to violate his oath of office so flagrantly.

“No president has ever used his office to compel a foreign nation to help him cheat in our elections -- prior presidents would be shocked to the core by such conduct and rightly so,” Rep. Nadler, D-N.Y., said in kicking off day two of the House impeachment managers' opening statements.

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“This conduct is not America First,” Nadler said, borrowing Trump’s campaign slogan. “It is Donald Trump first.”

Once Trump was caught pressuring Ukraine to interfere in the 2020 election by seeking investigations into Joe and Hunter Biden, he launched an unprecedented effort to stonewall Congress’ inquiry by denying documents and witnesses, Nadler charged.

“It puts even President Nixon to shame,” Nadler said.

Nadler's tough talk set the scene for the second of three days of the House managers' opening arguments against Trump, with a deep dive on the first article of impeachment, abuse of power.

On the first day of the opening arguments, Rep. Adam Schiff and his team used more than seven hours to make their case. That left 16 hours and 42 minutes on the opening statement clock between Thursday and Friday for the House Democrats. Then on Saturday, Trump's lawyers take the floor.

As the public case continued before the camera, another political campaign was underway behind the scenes over the issue of calling new witnesses. Democrats are seeking four GOP senators to join them in demanding new evidence in the trial, but Republicans are actively trying to avoid any GOP defections. No new witnesses would mean a speedy trial and a quicker vote to acquit the president.

Democrats are demanding the Trump administration cough up documents they've withheld related to Ukraine dealings and allow testimony from new witnesses who failed to show in the House inquiry, including former National Security Adviser John Bolton.

They were especially galled that Trump was in Davos, Switzerland, earlier this week and "gloat[ing]" that he as "all the material" and the Democrats don't, while warning that calling Bolton would be a national security risk.

“It is beyond belief -- beyond belief -- that the president of United States would even go to Davos in the middle of an impeachment trial, and then make statements like that to intimidate the jury, and to gloat over the fact that he has not turned documents over,” Sen. Tom Udall, D-N.M., said Thursday.

Some GOP senators have promoted witness reciprocity and called for Hunter Biden to testify about what he was doing on the board of the Ukraine energy company Burisma.

“Hunter Biden is a material witness, and if we're going to call witnesses, I want to see him on the stand and those questions need to be asked,” Sen. Joshua Hawley, R-Mo., said Thursday.

After opening statements from both sides, there will be another vote on whether to allow for new witnesses and documents in the coming days.

Hawley wouldn’t predict whether there would be GOP defections but said the caucus is in regular talks to keep tabs on where senators stand.

“There’s an ongoing conversation, informal, on what people are thinking. We are spending a lot of time together,” Hawley said of the marathon trial sessions.

Nadler will have to tread carefully Thursday. Chief Justice John Roberts admonished both sides Tuesday night after Nadler and White House Counsel Pat Cipollone tussled on the Senate floor.

Nadler accused Republican senators of engaging in a "cover-up" if they deny witnesses saying it's "an absolutely indefensible" and "treacherous" vote.

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His statements offended a key swing senator, Rep. Lisa Murkowski of Alaska. Hawley, who used to clerk for Roberts, said the admonishment was “extraordinary, especially for him.”

“He’s very understated. He’s very careful. … I was very surprised,” Hawley said of Roberts. "That was a pretty big slap, I thought, and it was deserved.”

The Senate GOP leadership is working to avoid going down the witness path with Democrats, according to a Senate leadership source. Leadership is actively reaching out to potential defectors: Murkowski, Sen. Susan Collins of Maine, Sen. Lamar Alexander of Tennessee and Sen. Mitt Romney of Utah.

The four were "instrumental" in changing McConnell’s organizing resolution on the fly to allow more days for opening statements and avoiding middle-of-the-night arguments. There is a strategic effort to keep these four involved in the process, the aide told Fox News.

A speedy trial and acquittal without witnesses and documents would benefit the White House and GOP leaders are trying to avoid a witness reciprocity showdown where Bolton would testify in exchange for, say, Hunter Biden.

“There’s a bunch of people on my side that want to call Joe Biden and Hunter Biden,” said Sen. Lindsey Graham, R-S.C. “I want to end this thing sooner rather than later. I don’t want to turn it into a circus."

Sen. Tim Scott, R-S.C., said it’s too early to tell whether the GOP will stay united on the matter of witnesses. So far, he found the House Democrats' arguments repetitive and “not necessarily effective.”

“Everybody wants all the things to stay together, but in the end it’s not really something we should comment on. We should just wait and see what happens," Scott said.

As Murkowski was darting to get the impeachment trial opening Thursday, she was asked if she was feeling any pressure. “Only to get upstairs,” she quipped before the elevator door closed.

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Inside the chamber, Democrats took pains on Thursday to debunk any allegations against former Vice President Joe Biden and his son Hunter for any wrongdoing. The White House said Trump was seeking the investigation from Ukraine to root out corruption, while Democrats said it was purely for political gain.

During her turn for arguments, Rep. Sylvia Garcia, D-Texas, accused Trump of cooking up the idea for an investigation after he saw Fox News polling that Biden was beating him in a 2020 matchup.

“There was no basis for the investigation that the president was pursuing and pushing. None. He was doing it only for his political benefit,” Garcia said.

Democrats held out some hope the House impeachment managers' arguments would shake loose some GOP support for witnesses and documents.

Sen. Sherrod Brown, D-Ohio, says he’s doing personal outreach to Republicans. Privately Republicans groan about the president’s conduct, but they are too scared of Trump to speak up, Brown said.

“I hear a number of Republicans telling me they know what the president did was wrong," Brown said. "Some of them say they know it’s really risen to the level of high crimes and misdemeanors … but they just simply aren't willing to vote that way because they're afraid of this president. They're afraid that president will call them names, will come to their state and campaign against them. And unfortunately fear does the business.”

Fox News’ Mike Emanuel, Adam Shaw and Hillary Vaughn contributed to this report.