Trump criticised . Rashida Tlaib of Michigan at the conservative youth gathering Turning Point USA on Tuesday, saying, "There's no way she stands for the values of the people of Michigan."

He also called her "vicious" and "a crazed lunatic".

Trump has been engaged in a back-and-forth with the women, following racist tweets he fired off suggesting they return to their countries, though all are American citizens and all but one born in the US

Turning Point USA describes itself as a youth organisation dedicated to educating and training and students "to promote the principles of freedom, free markets, and limited government".

It comes a Minnesota state legislator and a conservative group announced on Tuesday they are seeking ethics investigations into one of the other congresswomen, Ilhan Omar, for a range of alleged offences.

Steve Drazkowski, a Republican, and Judicial Watch both say Omar may have committed immigration fraud by marrying someone they say is her brother.

Omar, a Minnesota Democrat who came to the US as a refugee from Somalia, previously denied the allegation and called such claims "disgusting lies".

Drazkowski asked the House Ethics Committee to investigate; Judicial Watch's request went to the Office of Congressional Ethics.

They have cited social media posts, unidentified sources in the Somali-American community and address records to back up their claims.

Drazkowski, who said on Tuesday he had no prior knowledge of Judicial Watch's request, has filed successful complaints against Omar before.

Last month, Minnesota campaign finance officials found Omar violated state rules when she used campaign funds to pay for personal out-of-state travel while a state lawmaker.

They also found Omar and her husband filed joint tax returns years before they were legally married.

Omar was ordered to reimburse her former campaign committee nearly $US3500.

The requests for ethics inquiries come a week after President Donald Trump tweeted that Omar and the three other congresswomen should "go back" to their home countries.

All four women are US citizens, and Omar is the only one who wasn't born in the US.

Omar, one of the first Muslim women elected to congress, responded by tweeting that Trump was "stoking white nationalism".

Australian Associated Press