• Sen. Thom Tillis, R-N.C., called on Congress to investigate his GOP colleague Rep. Madison Cawthorn over allegations Cawthorn may have violated insider trading rules.
• The allegations from government ethics groups stem from Cawthorn's involvement with Let's Go Brandon coin, a cryptocurrency whose name is based on a conservative meme mocking President Joe Biden.
• Tillis' call for an investigation came as Cawthorn was already facing criticism from some within the Republican Party.

A Republican senator on Wednesday called on Congress to investigate his GOP colleague Rep. Madison Cawthorn, of North Carolina, who has come under fire as ethics watchdogs raise suspicions of possible insider trading over his relationship to an anti-Biden cryptocurrency.

"Insider trading by a member of Congress is a serious betrayal of their oath, and Congressman Cawthorn owes North Carolinians an explanation," Sen. Thom Tillis, also of North Carolina, wrote on Twitter.

"There needs to be a thorough and bipartisan inquiry into the matter by the House Ethics Committee," Tillis tweeted.

Tillis' tweet ratchets up the already intense pressure Cawthorn was facing from within his own party. The 26-year-old freshman lawmaker's recent comments and other troubles — including a citation Tuesday for bringing a loaded handgun into an airport — have made him a magnet for controversy as the GOP looks to regain majority control of the House in the midterm elections.

Tillis last month endorsed Cawthorn's primary challenger, and a Tillis-tied super PAC reportedly spent more than $300,000 on ads that attacked Cawthorn.

A spokesman for Cawthorn did not immediately respond to CNBC's request for comment. In an Instagram post Tuesday night, Cawthorn slammed "Rino senators and establishment pawns [who] want us to go back to the days before [former President Donald] Trump."

Tillis called for the ethics probe after government watchdogs told the Washington Examiner that Cawthorn may have broken federal insider trading laws, which restrict investors from profiting off nonpublic information.

Cawthorn has publicly boasted about his holdings of Let's Go Brandon coin, a cryptocurrency which gets its name from a conservative meme mocking President Joe Biden.

James Koutoulas, a lawyer and fund manager who is described in an investor lawsuit as the co-founder of the company, posted a photo on Instagram on Dec. 29 at a party with Cawthorn and Erik Norden, another co-founder of the cryptocurrency.

Cawthorn's official Instagram account commented on that photo: "Tomorrow we go to the moon!"

One day later, on Dec. 30, the racing team for NASCAR driver Brandon Brown announced that Let's Go Brandon coin had been signed as the team's primary partner for the 2022 season. Koutoulas is quoted in the press release.

Let's Go Brandon coin's value shot up more than 75% after the announcement, bringing the total value of all the coins in circulation to more than $570 million, according to news reports. Days later, NASCAR rejected the sponsorship deal, and by February the coin's price had plummeted.

The timeline of events suggests to some watchdog groups that Cawthorn may have had nonpublic knowledge of the deal with Brown at the time of the Dec. 29 Instagram post.

"It certainly raises some eyebrows," said Jordan Libowitz, spokesman for Citizens for Responsibility and Ethics in Washington, in a phone interview with CNBC.

He said it doesn't look good for Cawthorn, adding that the congressman's actions could pose ethical problems even if they do not violate the law.

"We tend to hope that our representatives in the government act on a higher level, that even the appearance of financial impropriety can sometimes be as bad as the impropriety itself," he said. "At the very least, Cawthorn has an optics issue, and at the worst there's a potential legal issue."

Koutoulas in a tweet Tuesday night accused Public Citizen of "libelous scumbaggery directed at" Cawthorn after a lobbyist for the group told the Examiner that Cawthorn may have violated insider trading rules.

Koutoulas, in a statement to CNBC, called the Examiner's report "absurd."

"It is not technically or legally possible for a decentralized meme coin that exists to promote free speech and charitable giving to be classified or treated as a security," he said.

An expert from the Project on Government Oversight, however, told the Examiner that even if the cryptocurrency is not classified as a security, insider trading rules could still apply.

In April, an investor in the coin filed a class-action lawsuit accusing the people behind the token of orchestrating a pump-and-dump scheme. The lawsuit does not name Cawthorn as a defendant but includes multiple instances of him promoting the coin. The legal complaint also alleges that after a similar version of the coin was relaunched in February, Cawthorn and others have continued to promote it.

Koutoulas' statement said the lawsuit's pump-and-dump allegations are "ludicrous," and slammed the plaintiff as a "crazed stalker."

Cawthorn recently has been in hot water for driving without a valid license and for claiming that other members of Congress were using drugs and inviting him to orgies. House Minority Leader Kevin McCarthy, R-Calif., has said there was no evidence for Cawthorn's claims.