President Obama charged Tuesday that Donald Trump is "unfit to serve" and "woefully unprepared" to do the job as president.

"I said so last week and he keeps on proving it," Obama said at the White House. "The notion that he would attack a Gold Star family that had made such extraordinary sacrifices on behalf of our country, the fact that he doesn't appear to have basic knowledge around critical issues, in Europe, in the Middle East, in Asia, means that he's woefully unprepared to do this job."

Obama was referring to Khizr Khan, the father of a U.S. soldier who died in 2004 and has been feuding with Trump for the last several days.

One of the main reasons Obama said Trump is unfit is that "a lot of people depend on the White House getting stuff right," he said with a pause for added emphasis.

Obama said prominent Republicans share his negative views on Trump, and called on them to withdraw their endorsements of Trump or risk years of fallout for themselves and the GOP as a whole.

"This is not just my opinion," he said. "[There have been] repeated denunciations of his statements by leading republicans – including the Speaker of the House and the Senate majority leader and prominent Republicans like John McCain. And the question, I think, they have to ask themselves is, if you are repeatedly having to say in very strong terms that what he has said is unacceptable, why are you still endorsing him? What does this say about your party that this is your standard bearer?"

Republican efforts to mitigate the impact of Trump's remarks isn't just a periodic problem, the president said.

"This is daily and weekly that they are distancing themselves from statements he is making," he said. "There has to be a point at which you say, this is not somebody I support to be president of the United States, even if he purports to be a member of my party. The fact that that has not yet happened makes some of these denunciations ring hollow."

If Republicans continue to stick by them as their nominee, they risk looking like "the entire Republican Party effectively endorses and validates the position being articulated by Mr. Trump," which he said could have serious repercussions for the party for years.