Though Barr has launched an exploratory committee to consider a White House bid as a Libertarian, many remember Barr as a vocal House Republican in the late 1990s, according to his former colleague Rep. Nathan Deal (R-Ga.).

Columnist George Will went one step further. “Come November, Barr conceivably could be to John McCain what Ralph Nader was to Al Gore in 2000 — ruinous,” Will wrote in his Newsweek column this week.

It’s Barr’s record that causes conservatives concern. During his eight years in Congress, Barr sponsored the Defense of Marriage Act, which allowed state same-sex marriage bans and is now law. He led an unsuccessful effort to repeal the assault weapons ban. And Barr famously introduced articles of impeachment against Clinton and served as a House manager in Clinton’s 1998 Senate trial.

Barr could get “a lot of voters who are disenchanted with government in general,” Deal said. “Potentially, he could pick up Ron Paul supporters.”