Sen. John McCain has taken some heat after tweeting a joke in which he compared Iranian President Mahmoud Ahmadinejad to a "monkey" after Ahmadinejad reportedly said today that he wanted to be the country's first man in space.

For McCain, the most prominent backlash has come from Republican Rep. Justin Amash, who is slamming McCain on Twitter:

Whoa.“@senjohnmccain: So Ahmadinejad wants to be first Iranian in space - wasn't he just there last week? "Iran launches monkey into space"” — Justin Amash (@repjustinamash) February 4, 2013

@_freeradical I can't believe he hasn't deleted it & apologized yet. — Justin Amash (@repjustinamash) February 4, 2013

Maybe you should wisen up & not make racist jokes. “@senjohnmccain: Re: Iran space tweet - lighten up folks, can't everyone take a joke?” — Justin Amash (@repjustinamash) February 4, 2013

For his part, McCain has told his critics to "lighten up," saying it was just a "joke."

The 31-year-old Amash is seen as an up-and-coming libertarian conservative in the mold of Ron Paul. Amash has been endorsed by Paul and has aligned his views with the former Texas Congressman.

Amash has also been a prominent member of the House GOP's more conservative group that has publicly challenged House Speaker John Boehner on recent budget battles.