Trump's lawyers say he's no longer 'impeachable.' Trump called for impeaching Obama in 2020.

At Donald Trump's second impeachment trial, a core argument made by the former president's legal team, and by Republicans, is that the trial is unconstitutional because Trump is no longer in office.

But at campaign rallies amid his reelection push, Trump himself repeatedly targeted former President Barack Obama, calling for his predecessor to be impeached despite being out of office for years, even joking about a "belated impeachment."

Arguing on the Senate floor on Tuesday during the first day of Trump's second impeachment trial, lawyer David Schoen forcefully argued that the trial was unconstitutional.

"Presidents are impeachable because presidents are removable. Former presidents are not, because they cannot be removed. The Constitution is clear," Schoen said. "Trial by the Senate sitting as a court of impeachment is reserved for the president of the United States, not a private citizen who used to be president of the United States."

In a brief submitted on Feb. 2 ahead of the trial, Trump's newly appointed legal team stated the trial is unconstitutional: "Since the 45th President is no longer 'President,' the clause 'shall be removed from Office on Impeachment for ...' is impossible."

The argument also has emerged as a central theme for Republicans and a way to focus on the process of the trial and avoid addressing whether or not Trump committed impeachable offenses tied to the Capitol attack. Last month, 45 Republican senators voted to dismiss the trial as unconstitutional on the grounds Trump is no longer president, including Senate Majority Leader Mitch McConnell, who said Trump "provoked" the mob that attacked the Capitol.

Facing his first impeachment trial for allegedly pressuring Ukraine for dirt on political opponents, Trump lashed out at Obama at multiple rallies around the country, calling for the former president to be impeached for promising "you can keep your doctor" -- referring to the Affordable Care Act -- despite having left office years earlier.

The night Congress voted to impeach Trump, Dec. 18, 2019, Trump first called for the former president to be impeached at a campaign rally in Battle Creek, Michigan.

"Remember Obama, 28 times? 'You can have your doctor, you can have your plan.' It did not work out that way, did it? I think we should impeach him for that. Let's impeach him. For that, for the IRS scandal, for the guns? Remember the guns?" Trump said at the packed rally. "Impeach him."

After being acquitted in the Senate in early February 2020, Trump again called for Obama to be impeached at a rally on Feb. 20 in Colorado Springs, Colorado, saying: "We should impeach him. We should impeach him. Impeach Obama."

The former president made similar statements a few days later at a rally in North Charleston, South Carolina, and then later that same month, at a rally in Las Vegas, Trump called for a "belated impeachment" of Obama before walking back the comment and saying it was a joke.

"Remember President Obama? He said, 'You can keep your doctor, keep your plan.' He lied. So we should impeach him. We should impeach him. We are going to impeach him. We will do a belated impeachment," Trump said. "We will do a belated impeachment."

"You have to be very careful when you say 'impeachment' because they go back and they write," Trump said, referring to the media. "I'm just kidding, OK?"

Representatives for Trump didn't respond to requests for comment on this story.

At the start of Tuesday's trial, Rep. David Cicilline, D-R.I., offered a pre-rebuttal for some arguments anticipated from former Trump's legal team, including the claim that the trial is now unconstitutional.

"The former president who promised on a Bible to use his power faithfully can and should answer for whether he kept that promise while bound by it in office," Cicilline said. "His insistence otherwise is just wrong. And so is his claim that there's a slippery slope to impeaching private citizens if you proceed."