Rand Paul shared "a few thoughts" on national security in a series of tweets Friday, including his belief that "Marco Rubio has failed" in his duty to protect Americans.

"I have a few thoughts on what true national security is, how we achieve it, and the abysmal record of one of my opponents in this area," the Kentucky senator and Republican presidential candidate tweeted at approximately 11 a.m. ET.

The feisty libertarian, who Rubio dubbed a "committed isolationist" during the fourth GOP debate, proceeded to rail against his colleague and 2016 opponent in nearly a dozen tweets, some of which promoted Paul's recent legislative proposal to ban refugees from 32 "high-risk" Muslim-majority countries. Rubio was among those who voted against Paul's amendment, which failed 89-10 to pass the Senate Thursday evening.