(CNN) Conservative radio host and former Illinois US Rep. Joe Walsh will challenge President Donald Trump for the Republican presidential nomination in 2020, he announced Sunday.

"I'm going to run for president," Walsh said on ABC's "This Week," also telling host George Stephanopoulos, "I'm going to do whatever I can. I don't want him (Trump) to win. The country cannot afford to have him win. If I'm not successful, I'm not voting for him."

Walsh had said Thursday that he was "strongly, strongly considering" entering the race.

"I'm not trying to be cute or coy. I've told you before -- if somebody's going to get in there and go after him ... it's got to be done soon," Walsh told CNN's John Berman on "New Day." "You're running out of time. But more importantly, these are not conventional times. Look at the guy in the White House. These are urgent times."

On Sunday, Walsh called Trump "nuts," "erratic," "cruel" and "incompetent."