President Barack Obama told a Hispanic audience on Thursday that his inability to overhaul U.S. immigration laws was the biggest shortcoming of his first four years in office, but he blamed Republican lawmakers for standing in the way.

"As you remind me, my biggest failure so far is we haven't gotten comprehensive immigration reform done," Obama said in an interview with Univision, the Spanish-language television network.

"But it's not for lack trying or desire," he added. "Obviously the fact that we haven't been able to change the tone in Washington is disappointing."

Obama was grilled about why he had not lived up to a promise he made in the 2008 presidential campaign to pass comprehensive immigration reform.

"At the beginning of your government, you had control of both chambers of Congress, and yet you did not introduce immigration reform," news anchor Jorge Ramos told the president. "I want for you to acknowledge that you did not keep your promise."

The president replied that arresting the nation's economic free fall had been his overriding priority in his first year in office. Efforts to lay the groundwork for immigration legislation stalled because of Republican indifference, he said.

"What we could not get was a single Republican, including the 20 who had previously voted for comprehensive immigration reform, to step up," Obama said.

Obama said during the interview on Thursday that despite political divisions in the capital, strong public pressure could lead to immigration reform.

"You can't change Washington from the inside, you can only change Washington from the outside," he said. "That's how the big accomplishments like health care got done, because we mobilized the American people to speak out."

Republican presidential nominee Mitt Romney jumped on that comment, saying it was an admission of failure by Obama.

"The president today threw the white flag of surrender again," the former Massachusetts governor said in Sarasota. "His slogan was 'Yes we can,' his slogan is now, 'No, I can't.'"

Immigration is a flashpoint in Obama's battle with Romney to win the White House on November 6.

Many conservatives advocate much tougher laws against illegal immigrants, and Romney has advocated what he calls self-deportation for illegals, stirring the ire of many Hispanics.

Obama, adopting a more lenient position, in June issued an order that allows hundreds of thousands of illegal immigrants who were brought to the United States as children to avoid deportation and obtain work permits.

While opponents decried the move as a blatant political ploy to win favor among Hispanics, Obama said on Thursday he already held a large advantage among Hispanic voters.

He said he was motivated to take that step because "there is no way you would think it was fair or just" for many young people to face deportation.

The move has allowed the president to draw a stark contrast between himself and Romney.