Paul first said he was "very worried about getting involved in a new war in Syria" as Syrian President Bashar Assad is a "bad guy."
Then Paul referred to McCain's recent trip to Syria where McCain was photographed with two men who were, according to Beirut news reports, involved in kidnapping a group of people from Lebanon.
"They say ‘there are some pro-Western people, and we’re going to vet them,’ well apparently we had a senator over there who had his picture taken with some kidnappers, so I don’t know how good a job we’re doing vetting those who are going to get the arms," said Paul on Saturday, during a Ronald Reagan Presidential Foundation forum.
Paul's comments were met with applause.
A number of people at the meeting where the photo was taken said the reports in the local press were wrong.
In the speech Paul also argued that the Republican Party has to attract a wider swath of the American electorate.
Earlier in the year, McCain referred to Paul and Sen. Ted Cruz (R-Texas) as "wacko birds" after the two senators filibustered John Brennan's nomination to run the CIA unless the Obama administration clarified its position on drone policy. McCain eventually apologized for the remark.