Though President Donald Trump has shrugged off calls for his impeachment, he has continued to attack the special counsel's Russia investigation on social media.

At the center of Republican attacks is the former White House counsel Don McGahn, who told investigators Trump had ordered him to fire the special counsel Robert Mueller.

House Speaker Nancy Pelosi is opposed to impeaching the president, but the Democratic presidential challengers Elizabeth Warren and Kamala Harris support it.

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President Donald Trump on Sunday said he was not "a little bit" worried by calls for his impeachment that rumbled over the weekend.

But reports and the president's social-media attacks indicate that the issue is very much on his mind.

According to The New York Times, Trump has expressed concerns about impeachment behind closed doors and worked out an aggressive strategy to respond.

Citing two sources close to the White House, The Times reported Tuesday that Trump thought the only way to protect himself from impeachment would be to attack the special counsel Robert Mueller and the former White House counsel Don McGahn. Thursday's release of Mueller's report into Russian election meddling and possible obstruction of justice by Trump revealed the extent to which McGahn cooperated with investigators.

Trump, according to The Times, is not moved by concerns that attacking McGahn could backfire by highlighting McGahn's testimony, which included claims that Trump tried to have Mueller fired.

With the release of the redacted report reigniting debate among Democrats over whether to file articles of impeachment against the president, details provided by McGahn in the report are central to the arguments of those who have called for the president to be impeached.

One side of the debate argues that Democrats should pursue claims that Trump obstructed justice. The other says Democrats should refocus, citing concerns that dragging on the fight could turn off voters in 2020.

Two Democratic presidential challengers, Sens. Elizabeth Warren of Massachusetts and Kamala Harris of California, are among those who have openly championed calls for the president's impeachment, but in a phone call with House Democrats on Sunday, House Speaker Nancy Pelosi opposed the move.

Trump's recent social-media activity seems to substantiate the idea that his response to impeachment talk is to lash out.

Don McGahn, then the White House counsel, behind Trump during a Cabinet meeting at the White House on June 21. REUTERS/Jonathan Ernst

"Only high crimes and misdemeanors can lead to impeachment," the president tweeted Monday, amid a flurry of retweets of allies and Fox News hosts attacking the Mueller investigation.

"There were no crimes by me (No Collusion, No Obstruction), so you can't impeach. It was the Democrats that committed the crimes, not your Republican President! Tables are finally turning on the Witch Hunt!"

The tweet marks a shift from his earlier claims of "total exoneration," which he made after Attorney General William Barr submitted a summary of the report to Congress in March.

Sen. Lindsey Graham of South Carolina — a key congressional ally of Trump's — predicted during a Monday interview with Fox News' Sean Hannity that Democrats would seek to impeach Trump.

He said such a move would strengthen the president and boost his chances of reelection in 2020.

"They're going to be stampeded to impeach Trump because they hate him so much, and I hate it for the country, I hate it for the president — but it's going to result in him getting reelected," Graham said.

Senate Republicans — whose support Democrats would need to remove Trump from office — have stuck by the president following the release of the report and hailed it as great news for his 2020 campaign.