MIAMI — US President Barack Obama faced a grilling before Hispanic voters Thursday on his failure to honor his promise to pass a bill to bring 11 million illegal immigrants out of the shadows.

Obama, at a forum hosted by the Univision Spanish language television network, admitted that he had fallen short on comprehensive immigration reform partly because he spent so much staving off a second Great Depression in 2009.

He also professed that the president is not all powerful in the US political system, and only represents one branch of a tri-cameral government, so needs help to accomplish his goals.

"What I confess, I did not expect -- and so I'm happy to take responsibility for being naive here -- is that Republicans who had previously supported comprehensive immigration reform ... suddenly would walk away," he said.

At the end of the forum, a day after a similar event featuring his Republican foe Mitt Romney, Obama was asked to name his biggest failure as president by one of the news anchors conducting the interview.

"As you remind me, my biggest failure so far is we haven't gotten comprehensive immigration reform done yet," Obama joked, sparking laughter from his audience.

The passage of a comprehensive immigration reform law in the US Congress is hugely complicated, as it involves a process that can be demagogued by opponents as "amnesty" for illegal immigrants.

It also requires compromise on such politically explosive issues as border security, guest workers and employment visas at a time when many native born Americans are struggling to find work.

Obama however said he hoped that if he wins a second term on November 6, Republicans would still reconsider their position and join in a drive to pass immigration reform.

He also brought up his order earlier this year to defer deportations of undocumented immigrants brought to the United States as children, which could benefit up to 1.7 million people, as proof of his commitment to the Hispanic community.

The initiative was criticized by Romney, who has also pledged to veto an bill known as the DREAM Act, which would go further for young illegal immigrants, if he becomes president.

"Whose vision best represents the aspirations not just of the Latino community, but of all Americans who believe that we are a nation of laws and a nation of immigrants? " Obama said.

"That candidate, I believe, is talking to you right now."

According to a recent Latino Decisions poll, Obama holds a huge lead, 68 to 26 percent, among Hispanic voters who are an increasingly important demographic in key swing states including Nevada and Colorado.

The question for Obama is whether Latinos will turn out in large numbers, amid dismay over his failings on immigration reform and the slow economic recovery which has hit the community hard.