The politicians’ dueling played out, as it almost always does these days, largely on Twitter and cable TV.

Rep. Ted Lieu (D) alleged in December that fellow California Rep. Devin Nunes (R) conspired with Lev Parnas, a former associate of President Trump’s personal lawyer Rudolph W. Giuliani, to undermine the United States. Parnas has pleaded not guilty to violating campaign finance laws.

Then a lawyer for Nunes, who is the ranking Republican on the House Intelligence Committee, sent a multi-page missive threatening to sue for damage to Nunes’s reputation, Lieu tweeted. The Democratic congressman replied with a letter of his own and posted a photo of the document online.

“I welcome any lawsuit from your client and look forward to taking discovery of Congressman Nunes,” he wrote. “Or, you can take your letter and shove it.”

On MSNBC’s “Hardball” on Friday, Lieu doubled down. “It turns out that based on text messages in the record and the amazing interview on [MSNBC’s] Rachel Maddow Show that I’m right,” he said. “Truth is a defense.”

Lieu, Nunes and Steven Biss, the attorney who said he was representing Nunes in the letter tweeted by Lieu, did not immediately respond to a request for comment on Friday night.

The squabbling is an unusual escalation of a clash between members of Congress. Although Nunes has shown himself to be litigious, his lawsuits typically target news organizations, Silicon Valley technology companies and, once, members of his own constituency.

Critics of Nunes’s defamation claims have said they are meant to stifle free speech on public issues. More than half of states have statutes intended to safeguard residents’ First Amendment rights in the face of these legal claims, called strategic lawsuits against public participation, but levels of protection vary.

The Constitution’s speech and debate clause protects comments made by members of Congress during speeches and debates in the chambers, but it is not clear whether that protection can be claimed to extend outside the House.

Nunes’s explanation of his level of familiarity with Parnas has changed over time. In November, an account from Parnas was a primary source for a CNN story reporting that Nunes had talked with former Ukrainian prosecutor general Victor Shokin about finding damaging information on former vice president Joe Biden, a political rival of Trump’s. Nunes responded by calling Parnas “a fraudster and a hustler.”

Then the House Intelligence Committee published its report on the impeachment inquiry into Trump, which centered on his interactions with Ukraine. The document showed phone calls between Nunes and Parnas. In response, Nunes told Fox News’s Sean Hannity that it was “possible” that he had spoken with Parnas but that he did not recall the name.

On Wednesday, Nunes acknowledged that he had talked with Parnas. He told Fox News host Martha MacCallum that he had reviewed his records and realized that he did have a “random” conversation with Parnas.

“I just didn’t know the name — this name ‘Par-nas,’ ” Nunes said. “You know now that he had called my cellphone, and I didn’t know his name.”

A few hours later, an interview with Parnas aired on MSNBC. Parnas claimed that Nunes was “involved” in pressuring Ukraine to dig up dirt on Biden and his son, Hunter Biden. “He knows who I am,” Parnas said of Nunes.

House Democrats on Friday released text messages between Parnas and Derek Harvey, an aide to Nunes, showing that Nunes’s office knew about the efforts in Ukraine and wanted to use the information that Parnas sought.

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