Joe Walsh, a former Republican congressman from Illinois and conservative radio host, has officially announced a challenge to Donald Trump for their party's 2020 presidential nomination.
Mr Walsh made the announcement on Twitter, where he specifically attacked Mr Trump, whom he had previously supported in 2016.
"We can't take four more years of Donald Trump. And that's why I'm running for President. It won't be easy, but bravery is never easy. But together, we can do it," Mr Walsh wrote in his campaign announcement.
The former Republican congressman supported Mr Trump in 2016, but said at the time that he only did so because the future president was not Hillary Clinton.
Since he became president, however, Mr Walsh said the president's behaviour has become increasingly concerning. The moment of no return came when Mr Trump, during a press conference last summer with Vladimir Putin, said that he trusted the Russian leader's words over American intelligence agency analysis of meddling in the 2016 election.