A Democratic impeachment witness apologized Wednesday after she name-dropped 13-year-old Barron Trump during the House Judiciary Committee hearing - and was slammed by First Lady Melania Trump.

Pamela Karlan, a Stanford University law professor, had been asked to describe the differences between a U.S. president and a king when she brought up the first son's name.

'The Constitution says there can be no titles of nobility, so while the president can name his son Barron, he can't make him a baron,' Karlan told lawmakers.

But about two hours later, Melania Trump tweeted a furious rebuke from Air Force One as she returned with her husband from the NATO summit in London.

'A minor child deserves privacy and should be kept out of politics. Pamela Karlan, you should be ashamed of your very angry and obviously biased public pandering, and using a child to do it,' tweeted the first lady.

The dispute came in the later stages of the Judiciary Committee's first hearing, a marathon when began at 10am and ended shortly before 7pm.

Democrats had wanted it to be a calm outlining of the constitutional basis for impeaching Trump by distinguished law professors, with three of the four witnesses backing up their case and even the Republican witness saying Trump's actions in Ukraine warranted investigation.

But when Karlan - a fiery testifier already - made her ill-fated joke about Barron Trump, the hearing took on a bitterly personal tone.

Soon after FLOTUS' rare rebuke on Twitter, Melania Trump's words were read aloud at the hearing, as it stretched into almost the eighth hour.

Several minutes later, Karlan - a Stanford constitutional law professor - apologized.

'I want to apologize earlier for what I said earlier about the president's son,' she said, noting she was happy to admit when she was wrong.

'I wish the President would apologize, obviously, for the things he's done that's wrong but I do regret having said that,' she added.

The first lady chimed in via tweet, calling out Stanford University law professor Pamela Karlan, one of the Democrats' three impeachment witnesses

Stanford law professor Pamela Karlan used Barron Trump as an example when she tried to describe the differences in powers between a president and a king

First lady Melania Trump took offense at one of the Democrats' witnesses name-dropping 13-year-old Barron Trump during Wednesday's impeachment hearing on Capitol Hill

First lady Melania Trump (right) made it clear Wednesday that 13-year-old Barron Trump (left) is off limits

Donald Trump Jr., Barron Trump's oldest brother, also slammed Democratic witness Pamela Karlan, over her reference to Barron during Wednesday's impeachment hearing

Donald Trump Jr., the president's oldest son, also chimed in.

'Remember only children of Liberals (even the 50 year old children) are off limits. That a 13 year old is fair game for partisan hacks on a national platform shows you all you need to know about the new left and who is now running the ship,' the Trump son tweeted.

He was likely referencing Hunter Biden, the 49-year-old son of former Vice President Joe Biden, whose business dealings in Ukraine created the impeachment snowball.

Prior to the first lady's reaction, the White House's Twitter account commented, 'Democrats are calling as witnesses the kind of people who would name the minor child of the President as a punchline in a committee hearing.'

The tweet showed a video of Rep. Matt Gaetz, a Florida Republican, who tore into Karlan during Wednesday's Judiciary Committee impeachment hearing, which was a first for that particular committee.

'Let me also suggest that when you invoke the president's son's name here, when you try to make a little joke out of referencing Barron Trump, that does not lend credibility to your argument, it makes you look mean,' Gaetz said. 'It makes you look like you're attacking someone's family - the minor child of the president of the United State's family.'

Gaetz also went after Karlan for making Democratic political donations, including giving $2,000 to President Trump's 2016 rival, Hillary Clinton.

Gaetz asked why Karlan had given so much to Clinton and not subsequent Democratic candidates.

'Because I've been giving a lot of money to charity recently because of the poor people in the United States,' she replied, a clear dig at GOP policies.

Donald Trump Jr. also pointed to Karlan's Democratic donations.

'I guess this explains why this partisan "scholar" it would go on national TV and try to make a joke out of a 13-year-old child. Obviously no bias here,' he wrote. 'GTFO with this nonsense already.'

The exchanges between Karlan and Gaetz were one of the most heated between lawmakers and witnesses in a hearing marked by Republican interruptions, Chairman Jerry Nadler clanking his gavel and partisan opining.

Republican Rep. Ken Buck had one of the most colorful defenses of President Trump, arguing that under Democrats' standards, every president was impeachable.

'Lyndon Johnson directed the Central Intelligence Agency to place a spy in Barry Goldwater's campaign. Would that be impeachable conduct according to the other panelists?' Buck asked. 'How about when President Kennedy directed his brother Robert Kennedy to deport one of his mistresses as an East German spy, would that qualify as impeachable conduct?'

Rep. Ken Buck, a Colorado Republican, tried a new way to dash Democrats' push for impeachment - he used examples from other presidents, suggesting they all committed impeachable acts under Democrats' standards

Judiciary Committee Chairman Jerrold Nadler kicked off the hearing by linking President Trump's Ukraine pressure campaign, which is central to the current impeachment inquiry, to Russia and the Mueller Report

House Judiciary Committee Chairman Jerrold Nadler (left) talks with the committee's top Republican, Rep. Doug Collins (right)

Buck was talking about Ellen Rometsch, who Robert Kennedy biographer Evan Thomas said was deported to cover up her affair with the president.

The Colorado Republican also brought up a favorite Republican talking point: Benghazi.

And even name-dropped the first Republican president, Abraham Lincoln, pointing to his arrest of Maryland legislators to crush the state's Confederate rebellion.

'How about when the president directed his national security adviser and the secretary of state to lie to the American people about whether the ambassador to Libya was murdered as a result of a video or was murdered as a result of a terrorist act, would that be an abuse of power to a political benefit, 17 days before the next election?' Buck asked, referencing Obama, and his actions on the heels of the September 11, 2012 Benghazi attack, which came about six weeks before Obama was on the ballot again.

Buck directed his questions at George Washington University law professor Jonathan Turley, the one GOP-called witness at the Judiciary Committee hearing.

Turley mostly played along and said that in most of these instances, the other three law professors who were Democratic witnesses would consider these actions impeachable offenses.

'Can you name a single president in the history of the United States, save President Harrison who died 32 days after his inauguration, that would not have met the standard of impeachment for our friends here?' Buck asked.

Turley made a joke when he gave his answer, 'I would hope to God James Madison would escape, otherwise a lifetime of academic work would be shredded,' said the Madison expert. 'Once again, I can't exclude any of these acts.'

Republicans started early in trying to trip up Wednesday's hearing, interrupting Judiciary Chairman Jerry Nadler and forcing several committee votes.

They asked that the so-called 'whistleblower' be subpoenaed. They also pushed for House Intelligence Committee Chairman Adam Schiff, the California Democrat who's been leading the impeachment effort thus far, to testify. As the minority, the GOP lost.

Rep. Louie Gohmert called for a trio of new witnesses, including two National Security Council staffers who later went to work for Schiff.

All the while, Nadler banged his gavel and pushed through Republican protests.

'Thanks for bringing down the gavel hard, that was nice,' Gohmert commented at one point, as Nadler cut him off.

The New York Democrat gave an opening statement linking President Trump's pressure campaign in Ukraine - the heart of the impeachment matter - to the president's conduct related to Russian interference.

'Of course this is not the first time that President Trump has engaged in this pattern of conduct,' Nadler said, bringing up Russia's campaign in 2016 to interfere in the U.S. election. 'President Trump welcomed that interference.'

Nadler also accused Trump of taking 'extraordingary and unprecedented steps to cover-up his efforts,' speaking about the lack of witnesses and documents the White House provided and reminding the audience that Bill Clinton, when he was under investigation and was impeached, literally donated 'blood' to the probe.

'No other president has vowed to 'fight all of the subpoenas' as President Trump has promised,' Nadler said. 'In the 1974 impeachment proceedings, President Nixon produced dozens of recordings. In 1998, President Clinton physically gave his blood. President Trump, by contrast, has refused to produce a single document and directed every witness not to testify.'

Rep. Doug Collins, the top Repubilcan on the committee, used his opening statement to call the proceedings a 'sham'

'So don't tell me this is about new evidence and new things and new stuff,' he said. 'This is nothing new folks,' he added, pointing out that Nadler had started talking about impeaching Trump a year ago - long before the Ukraine scheme was first uncovered.

Turley, the only Republican witness allowed by Democrats to appear at the impeachment hearing Wednesday, did not use his opening statement to defend Trump.

The George Washington University law professor admitted he didn't vote for the president.

'I'm not a supporter of President Trump. I voted against him,' he said during his opening statement before the House Judiciary Committee, claiming it was an irrelevant fact. 'My personal views of President Trump are as irrelevant to my impeachment testimony as they should be to your impeachment vote.'

Turley said that while 'a case for impeachment can be made,' the current case brought by Democrats was based solely on secondhand information.

'I am concerned about lowering impeachment standards to fit a scarcity of evidence,' Turley continued in his remarks.

He blasted the president's call with his Ukrainian counterpart as 'anything but perfect,' – a word Trump has used to describe his now-infamous July 25 call with Ukrainian President Volodymyr Zelensky, which is the genesis of the impeachment inquiry.

Turley also described the current period as one of 'madness.'

'I get it, you're mad,' Turley said in his remarks aimed at the Judiciary panel. 'The president's mad. My Republicans friends are mad. My Democratic friends are mad. My wife is mad. My kids are mad. Even my dog seems mad – and Luna's a Golden Doodle, and they don't get mad. So we're all mad.'

'Will a slip-shot impeachment make us less mad?' he posed.

'It's not wrong because President Trump is right,' Truley said of the impeachment proceedings. 'His call was anything but perfect. It's not wrong because the House has no legitimate reason to investigate the Ukrainian controversy. It's not wrong because we're in an election year - there is no good time for an impeachment. No, it's wrong because this is not how you impeach an American president.'

The remaining three witnesses invited to publicly testify on Wednesday were all called by Democrats and included Noah Feldman, a Harvard Law professor, Karlan and Michael Gerhardt, a law professor at the University of North Carolina.

They all argued for impeachment in their opening statements before the panel.

'I just want to stress, that if this – if what we’re talking about is not impeachable, than nothing is impeachable,' Gerhardt said in his uninterrupted remarks. 'This is precisely the misconduct that the framers created a constitution – including impeachment – to protect against.'

'If Congress concludes that they’re going to give a pass to the president here… every other president will say, "Ok, then I can do the same thing." And the boundaries will just evaporate,' he continued. 'And those boundaries are set up by the Constitution, and we may be witnessing, unfortunately, their erosion. And that is a danger to all of us.'

Asked by Democratic counsel Norm Eisen if Trump committed the impeachable high crime and misdemeanor of abuse of power, they all answered in the affirmative.

'We three are unanimous,' Gerhardt said.

Eisen didn't ask Turley the question.

Later, Collins commented, ''Prof. Turley, you're now well-rested,' after Democratic counsel largely ignored him.

Witnesses in Wednesday's hearing included (from left) Harvard law professor Noah Feldman, Stanford law professor Pamela Karlan, University of North Carolina law professor Michael Gerhardt and George Washington University law professor Jonathan Turley

Prior to her Barron Trump comment, Karlan talked about how she had spent her entire Thanksgiving vacation going over the transcripts of the previous impeachment witnesses, joking that her turkey 'came to us in the mail that was already cooked.'

She said the 'most chilling line' came from testimony provided by E.U. Amb. Gordon Sondland who had he had never heard that the Ukrainians needed to go through with the investigations, just announce them publicly.

'This was about injuring someone who the president thinks of as a particularly hard opponent,' she said in reference to Joe Biden.

Feldman also briefly shamed lawmakers into action.

'Someday we will no longer be alive and we will go wherever it is we go, the good place or the other place and we may meet there Madison and Hamilton and they will ask us when the president of the United States acted to corrupt the structure of the republic, what did you do?' he began. 'And our answer to that question must be that we followed the guidance of the framers. And it must be that if the evidence supports that conclusion that the House of Representatives moves to impeach him,' Feldman said.

Earlier Wednesday, President Trump expressed outrage for the timing of the hearing, as he concluded his final day at he NATO summit in London.

'Frankly, it's a bad thing for the country,' Trump said. I'm over here at NATO. They scheduled it - the same thing happened four months ago when they put the United Nations - UNGA - they put the United Nations situation, they had a hearing with somebody on the same day.'

Trump was referencing a House Intelligence Committee hearing in September with the acting director of national intelligence Joseph Maguire, whose testimony on Trump's call with Zelensky led to the impeachment inquiry. At the time, Trump was attending the United Nations General Assembly in New York.

'It is the most unfair thing people have ever seen,' Trump griped.