(CNN) – Choosing who to vote for in a potential 2016 matchup between Democrat Hillary Clinton and Republican Rand Paul would be hard for Sen. John McCain, the Arizona Republican joked in an interview published Wednesday.
"It's gonna be a tough choice," McCain, laughing, told "The New Republic."
The 2008 Republican presidential nominee hasn't hidden his opposition to some of Paul's positions in the past, including the libertarian-minded senator from Kentucky's stance on drones and U.S. intervention in foreign conflicts.
Condemning Paul's 13-hour filibuster over the use of drones, McCain called his colleague, along with fellow Republican Sen. Ted Cruz, "wacko birds."
Other Republicans have also joined the back-and-forth – New Jersey Gov. Chris Christie and New York Rep. Peter King both chastised Paul for his opposition to government spying programs.
In response, Paul has hit back on those lawmakers' records on spending, saying they've been over-dependent on federal funds for things like disaster relief.
In an interview on CNN's "The Situation Room with Wolf Blitzer" Tuesday, Paul defended himself against Christie's most recent allegations that the senator was bringing home too much money for his state.
"This is the king of bacon talking about bacon," Paul said about Christie. Both are considered potential 2016 contenders for the Republican presidential nomination.
CNN's Kevin Liptak and Ashley Killough contributed to this report.