An outspoken senator, who has long-been critical of President Donald Trump, says that the Republican Party should have stopped his “birther” movement when it began.

Sen. Jeff Flake, R-Ariz., said in a televised interview that he regrets that his fellow Republicans stayed silent years ago when Trump pushed forth his theory that former President Barack Obama wasn’t born in the U.S., according to the New York Daily News.

“I wish that we, as a party, would have stood up...when the birtherism thing was going along,” Flake told NBC during the interview. “That was particularly ugly.”

Flake also called out the GOP for supporting Trump’s chants to “lock her up” about opponent Hillary Clinton during his campaign rallies, as well as for releasing what he said were false facts and conspiracy theories.

“We shouldn't be the party for jailing your political opponents,” Flake said.

“Anybody at those rallies ought to stand up and say, ‘That's inappropriate. We shouldn't be doing that.’ And I wish we as a party and elected officials would do more of that. Or when particular ugly conspiracy theories go out, or simply fake news, stuff that is just demonstrably false. We ought to stand up and say, ‘Hey, that’s just not right.’”

It was years before he began his presidential bid when Trump entered the political arena by publicly backing the “birther” movement which was largely ignored by the mainstream media until he got involved.

Trump was relentless in his questioning of Obama’s place of birth, forcing him to release his long-form borth certificate in April 2011.

Trump did not relent on his false allegations until September 2016, when he said that Obama “was born in the United States. Period.”

Sen. Flake has become one of the GOP’s most outspoken critics of the president after publishing a book earlier this month in which he takes the party to task for supporting Trump’s candidacy.