Immigration proponents worried about the future of reform in Congress have made clear their backup plan is to ask President Obama to use his executive authority to defer deportations for 11 million undocumented immigrants.

But one leading Republican advocate for reform said that would be a "big mistake."

Former Homeland Security Secretary Michael Chertoff believes a unilateral move by the White House would further doom the chances of Congress embracing sweeping reform legislation. Chertoff, who advocated for sweeping changes to immigration law under President George W. Bush, said Obama's election-year decision in 2012 to defer deportations for immigrants brought to the country illegally as children has caused great skepticism among reform opponents.

"I understand when the decision was made not to deport the so-called 'Dreamers' it was seen as very humane. But for people skeptical and distrusting, that vindicated their skepticism," Chertoff said during an immigration forum in Raleigh, N.C., co-sponsored by a pro-reform group called Bibles, Badges and Business, the North Carolina Farm Bureau and Bipartisan Policy Center.

Chertoff said that for opponents of reform to embrace a comprehensive bill, they must be convinced that the government will uphold all elements of any new law, including additional border security and workplace screening.

"They think, 'Well, great, we pass immigration reform, then once people are legalized, the president will waive all the requirements of border security and not enforce them,'" Chertoff said. That fear "really lurks behind the argument against this" immigration reform proposal.

Since Obama announced his deferred deportation program for younger immigrants a year ago, his administration has waived deportation proceedings for more than 400,000 people. Some Republicans, including Sen. Marco Rubio (R-Fla.), have said that the president is likely to expand that program if immigration reform fails.

But Chertoff said he was in a meeting with Obama a few years ago when the subject of deferred deportations came up. According to Chertoff, Obama said: "I can't do that unilaterally because we have to enforce the law."

"That was the right answer," Chertoff said.

Chertoff helped spearhead Bush's unsuccessful immigration push in 2007, when a bipartisan proposal failed to advance out of the Senate. He has since been supportive of Obama's push for reform.

This year, the Senate approved a comprehensive bill that includes a path to citizenship for the 11 million undocumented immigrants, but the GOP-controlled House has refused to vote on the measure. Leaders have said they will instead pursue a piecemeal approach focused on border security, which is unlikely to include a chance at citizenship for the immigrants.

Chertoff acknowledged that the prospects of immigration reform have dimmed recently as Congress's legislative window has grown more limited because of the debate over Syria, and looming battles over the budget and the debt ceiling.

"The other danger in this is that in the course of addressing these issues, so much bad blood is generated that their ability to work together and that trust erodes," he said. "The public has to send a message to members of Congress that they are paid to accomplish things. That does not mean you get everything you want and the other guy doesn't get anything."