Charles Krauthammer weighed in on the huge pool of possible 2016 presidential contenders for the Republican party tonight on "The O'Reilly Factor."

Bill O'Reilly asked Krauthammer his thoughts about each person who could possibly run for the White House, noting that Gov. Scott Walker got a big boost when Rush Limbaugh complimented him for the speech he made in Iowa this past weekend.  

Krauthammer said that it changes things for Walker in a positive way. 

"I think it does, but it was the speech itself that changed things for him," he said. "The ones who are serious are the ones who impress audiences. I think Scott Walker is getting up there to the top tier." 

Krauthammer also said he thinks it would be a mistake for Romney to run for president again. He remarked that if Jeb Bush "had a different last name, he would be perceived totally differently."

"You can be helped by your last name or hurt," he said. "For Bush, he's definitely hurt."  

Watch the video above to hear more.