James Carville. Sarah Jacobs/Business Insider James Carville on Tuesday delivered harsh criticism of Donald Trump's knowledge of key foreign-policy issues.

In an interview with Business Insider, President Bill Clinton's former campaign manager said that while he disagreed with former Republican presidential nominees like Sen. John McCain of Arizona and former Gov. Mitt Romney of Massachusetts on policy, they were far better versed in the details of governing than the current GOP nominee.

"We'd say in Louisiana, he don't know jack s---," Carville said of Trump.

"There's certain elementary knowledge [you need] to be in anything in politics, much less the president of the United States," he added.

Carville criticized both Trump's threat to start a trade war with China and his suggestion that the US could abandon NATO allies if they did not "fulfill their obligations" to meet certain requirements.

"For one minute, digest a 35% tariff — assuming that you'd ever put it on — but imagine you'd even threaten that," Carville said. "What would that do? How would the markets react to the fact that we're entering a trade war?"

Carville acknowledged that he found Trump "kind of funny," brushing off observers who point out that Trump draws larger crowds to his campaign events than Clinton draws to hers.

"Somebody said, 'Well, he gets bigger crowds than Hillary.' And my answer is, 'Yeah, more people line up to see a trainwreck than a traffic light,'" Carville said. "So what the hell does that mean?"

The Clinton campaign and its allies have frequently attempted to cast doubt on the real-estate magnate's ability to effectively manage US relationships abroad. Earlier this month the campaign rolled out a letter cosigned by numerous Republican foreign-policy experts denouncing Trump's policies and positive statements about Russia.

Trump has responded in kind, accusing Clinton and the Obama administration of neglecting to quash the unrest in Syria that led to the rise of ISIS.

"Hillary Clinton's reckless policies unleashed ISIS onto the world," Trump said in a Wednesday speech in Florida. "She will never have the judgment or the stamina to fix the problems she created."