The former US president and Republic contender attacks his Democratic opponents after Biden withdraws from the race.

Shortly after US President Joe Biden announced he would not run for a second term, his Republican opponent Donald Trump said that “crooked Biden wasn’t fit to run” for president and that Vice President Kamala Harris will be “easier to defeat” in November’s election.

“All those around him, including his Doctor and the Media, knew that he wasn’t capable of being President, and he wasn’t,” Trump wrote on his Truth Social network on Sunday.

“We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly. MAKE AMERICA GREAT AGAIN!” he added.

Biden, 81, ended his re-election campaign after fellow Democrats lost faith in his mental acuity and ability to beat Trump. The oldest sitting US president endorsed Harris to replace him as the party’s candidate.

In his first reaction to Biden’s announcement, Trump, in an interview with the CNN network, said he thinks Harris will be easier to defeat in the election.

Publicly, Trump campaign advisers and allies have been telling reporters they are not worried about facing Harris because they can simply tie her to Biden’s record in office, particularly on immigration and inflation.

They say they will try to portray Harris, and any of the other candidates being suggested as alternatives for the Democrats, as being to the left of Biden on various policies.

Trump’s oldest son, Donald Trump Jr, said on social media after Biden dropped out that Harris “owns the entire leftwing policy record of Joe Biden”.

“The only difference is that she is even more liberal and less competent than Joe, which is really saying something. She was put in charge of the border and we saw the worst invasion of illegals in our history!!!” he posted on X.

Some Republican Party leaders called for Biden to resign as US president.

“If Joe Biden can’t run for re-election, he is unable and unfit to serve as President of the United States. He must immediately resign,” Elise Stefanik, House Republican Conference chairperson, said in a statement.

For weeks, the Biden campaign had insisted that the Democratic leader was staying in the race. But after his decision to stand down from re-election, the Democratic Party has been plunged into uncertainty, less than four months ahead of the November election.

Jennifer N Victor, professor of political science at George Mason University in the US, told Al Jazeera that Trump’s modus operandi in politics “has always been to insult his political opponents”.

“I don’t take Trump as the historian in Joe Biden’s presidency,” she said.