Rand Paul is the worst of the score of Republicans running for the White House, his Senate colleague from Arizona, John McCain, told Fox News on Wednesday, the New York Daily News reported.

The Kentucky senator's earlier comments labeling McCain a "lapdog" for President Barack Obama's foreign policy apparently did not sit well with the 2008 GOP nominee.

"The record is very clear that he simply does not have an understanding about the needs and the threats of United States national security," McCain said about Paul. "In 2007, he said it would be ridiculous to call Iran a threat to the United States' national security. Last summer, he publicly doubted whether ISIS was a threat to the United States' national security," he detailed.

In his final analysis on the channel's "Your World With Neil Cavuto," McCain did not hold back, either, Fox News noted.

"Senator Paul is the worst possible candidate of the 20 or so that are running on the most important issue, which is national security," he insisted.

McCain's ire may have been prompted in part by remarks Paul made on Tuesday on Fox's "America's Newsroom," in which the candidate attacked the Arizonan and the latter's longtime Senate ally Lindsey Graham.

Asked about recent criticisms the duo had leveled against his foreign/policy outlook, Paul had charged that McCain and his South Carolina colleague "have been wrong about every foreign-policy issue over the last two decades."

"I'm really the one standing up to President Obama, and these people are essentially the lapdogs for President Obama and I think they're sensitive about that," he said.

Graham, who himself is considering jumping into the 2016 White House race, on Wednesday similarly dismissed the criticism, noting that Paul would have the "worst chance of anybody to make a case against Obama's foreign policy," according to Politico.

"At the end of the day, his record, in my view, shows a foreign policy vision one step behind leading from behind," the South Carolinian charged. "And all I can say is that if he's the nominee, I will support him. But if he's the nominee of the party, we risk giving up the central issue of the 2016 campaign, which will be foreign policy," Graham added.