Donald Trump has lashed out at Mark Sanford, the latest Republican to challenge the US president in the party’s primary contest.
Mr Trump described Mr Sanford, a former South Carolina governor, and two other hopefuls as “badly failed candidates”.
It is seen as near impossible that anyone will take the Republican mantle from the president.
No sitting president in the modern era has lost the race to be nominee for their own party, and Mr Trump remains very popular with Republicans.
In April, former Massachusetts Governor Bill Weld became the first person to challenge Mr Trump.
He was followed by conservative radio host and former lawmaker Joe Walsh at the end of August.
Mr Walsh has described President Trump as “narcissist”.
The Republican National Convention, at which the nominee will be formally chosen, will take place in late August 2020 after a series of state primary elections and party caucuses.
But some state Republican parties, including in South Carolina, have decided not to hold primaries in 2020 – to clear the path for Mr Trump and save money.
What did President Trump say?
In a series of tweets, Mr Trump recalled the scandal in June 2009 when Mr Sanford went missing for several days, with his staff telling reporters he had gone to hike the Appalachian Trail. Mr Sanford later admitted he had instead gone to Argentina to see his mistress.
Mr Trump described Mr Sanford’s mistress as his “flaming dancer friend”. It is unclear why he used these words, but this could be a reference to media reports that Mr Sanford first met the Argentine woman at an open-air dance.
Mr Trump also said his three potential rivals were “stooges, all badly failed candidates”.
What about Mr Sanford’s decision to run?
Mr Sanford, aged 59, announced that he would challenge Mr Trump during an interview with Fox News on Sunday.
“I think we need to have a conversation on what it means to be a Republican. I think that as a Republican party we have lost our way,” Mr Sanford said.
“We have lost our way on debts and deficits and spending… The president has called himself the king of debt, has a familiarity and comfort level with debt that I think is ultimately leading us in the wrong direction.”
Mr Sanford is expected to centre his campaign on cutting government debt and spending.
Who is Mark Sanford?
He first served in Congress in 1995, representing South Carolina’s first congressional district. He later served as the state’s governor for two terms from 2003-2011. He then returned to the House in 2013.
Mr Sanford criticised Mr Trump during the 2016 presidential election but ultimately supported him. However, he would become one of his toughest Republican critics in Congress when Mr Trump took office.
That stance cost him the Republican primary when his seat was up for re-election last year. He was beaten by a pro-Trump challenger who went on to lose the election to her Democrat opponent.
Who will take on Trump in 2020?
Election day is still more than a year away but the race to become the Democratic challenger to Mr Trump is already well under way.
Joe Biden and Bernie Sanders have thrown their hats into the ring, but most of the other candidates are relatively unknown outside the Washington DC bubble.