After a crowd booed GOP frontrunner Donald Trump for calling fellow Republican Sen. Marco Rubio (R-FL) a “clown,” Trump took to Twitter to double down on the attacks.

Fresh off an appearance at the conservative Values Voter Summit on Friday, Trump slammed Rubio on his record in the Senate.

.@MarcoRubio is weak on illegal immigration and will allow anyone into the country….. — Donald J. Trump (@realDonaldTrump) September 25, 2015

…..Has worst attendance record in Senate- rarely there to vote on a bill! @marcorubio — Donald J. Trump (@realDonaldTrump) September 25, 2015

According to the website GovTrack, Rubio has missed 11 percent of roll call votes from January 2011 to September 2015, compared to a 1.6 percent median lifetime record for senators currently serving.

The billionaire real estate mogul also went after Rubio, who is currently polling around 9 percent, for being a member of the so-called “Gang Of Eight,” the bipartisan group of senators who endorsed a path to citizenship in 2013.