Sen. Marco Rubio on Sunday blasted President Trump for announcing he’s working with Russian President Vladimir Putin on cyber security, saying such a deal is like partnering with Syrian President Bashar Assad on a “Chemical Weapons Unit.”

Rubio, R-Fla., made the comment in a series of tweets after Trump met personally with Putin last week and earlier Sunday tweeted: “Putin & I discussed forming an impenetrable Cyber Security unit so that election hacking, & many other negative things, will be guarded … and safe.”

Rubio, who has had an uneasy relationship with Trump at least since competing against him in the 2016 Republican presidential primaries, tweeted: “Partnering with Putin on a ‘Cyber Security Unit’ is akin to partnering with Assad on a "Chemical Weapons Unit.”

Assad has purportedly ordered chemical weapons attacks on civilians opposed to his regime, in the country’s years-long civil war.

Russia meddled in the 2016 White House race, according to members of the U.S. intelligence community.

Trump, during the 2016 primaries, called Rubio “Little Marco.” And in a largely final attempt to defeat Trump, Rubio insulted Trump by saying he had “small hands.”

On Sunday, Rubio also tweeted: “We have no quarrel with Russia or the Russian people. Problem is with Putin & his oppression, war crimes & interference in our elections,” and “While reality & pragmatism requires that we engage Vladimir Putin, he will never be a trusted ally or a reliable constructive partner.”

Treasury Secretary Steven Mnuchin was critical of Rubio's tweets.

“I think very highly of the senator," he told ABC's "This Week." "But … I think this is a very important step forward. What we want to make sure is that we coordinate with Russia, that we're focused on cybersecurity together, that we make sure that they never interfere in any democratic elections or conduct any cyber security. …I think is a very significant accomplishment for President Trump."

South Carolina GOP Sen. Lindsey Graham, another former 2016 presidential contender, was critical of the Trump-Putin idea.

"It's not the dumbest idea I've ever heard, but it's pretty close," he told NBC’s “Meet the Press. "The more you do this, the more people are suspicious about Russia.”