WASHINGTON – As a third Republican announced he would challenge President Donald Trump for next year's Republican presidential nomination, the incumbent took to his favorite social media forum Monday to mock all of his GOP opponents.

"The Three Stooges, all badly failed candidates, will give it a go!" Trump said in tweets aimed primarily at his latest challenger, former South Carolina governor and congressman Mark Sanford.

The president also presumably referred to his two other Republican primary challengers, former Illinois congressman Joe Walsh and former Massachusetts Gov. Bill Weld.

In singling out Sanford, Trump cited the former South Carolina governor's most famous moment: His travels to Argentina to see a girlfriend when he had told aides he was out hiking the Appalachian Trail. Sanford later won election to a U.S. House seat but lost last year to a Republican primary challenger endorsed by Trump.

"But now take heart, he is back, and running for President of the United States," Trump tweeted.

In challenging Trump for next year's presidential nomination, Sanford has criticized Trump for his insulting manner, and said he is turning women and young women away from the Republicans.

"I am compelled to enter the Presidential Primary as a Republican for several reasons – the most important of which is to further and foster a national debate on our nation's debt, deficits and spending," Sanford tweeted on Sunday.

Walsh and Weld have voiced similar criticisms of Trump, especially his volatility.

All face uphill battles in unseating Trump, however.

For one thing, Republicans are canceling presidential primaries in several states, making it very difficult for any challenger to get any kind of a foothold.