Sen. Marco Rubio, R-Fla., predicted Sunday that President Obama’s expected executive actions to address millions of immigrants living in the U.S. illegally would deal a major blow to legislative hopes for comprehensive immigration reform.

“That would be one step forward but three steps back,” Rubio, an immigration reform supporter, said on “Fox News Sunday,” calling the threat of executive action the “single greatest impediment to moving forward.”

Obama is expected to reveal his long-awaited unilateral actions around Labor Day. The White House is weighing whether to expand a previous order that deferred the deportations of younger illegal immigrants to millions of other people living in the U.S. illegally.

Obama said the need for executive action proved all the more evident with Congress unable to pass even a bill to address the surge in unaccompanied minors crossing the U.S.-Mexico border.

Rubio, who saw his own political fortunes take a hit after pushing a Senate immigration bill last year, insists that Republicans cannot ignore the issue.

“I knew how difficult it was politically. I didn’t get elected to maintain good poll numbers nationally,” he said on the Fox program.

“We’re not debating what to do,” he added. “We’re debating how to do it.”