WASHINGTON -- With frustration mounting, President Obama sought Thursday to quell doubts he'll use his presidential powers to act on immigration, telling Hispanics and immigration activists it's "not a question of if, but when."

At the Congressional Hispanic Caucus Institute's annual gala, Mr. Obama warned activists that his eventual actions will spark intense political opposition that could threaten the durability of what he does.

In a partisan pitch a month before Election Day, he urged Hispanics across the U.S. to use their votes to improve prospects in the future for a legislative fix.

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"The moment I act - and it will be taking place between the November election and the end of the year - opponents of reform will roll out the same old scare tactics," Mr. Obama said. "When opponents are out there saying who knows what, I'm going to need you to have my back."

Once hailed as a champion for Hispanic rights, Mr. Obama's relationship with the Hispanic community has become strained since he decided last month to abandon his earlier pledge to act quickly after summer's end to help some immigrants who are in the U.S. illegally. Instead, he said he'd wait until after the Nov. 4 elections, exasperating immigration activists who accused the president of putting politics ahead of their families and said they had waited far too long already.

With the elections nearing, Mr. Obama sought to parlay impatience into motivation for Hispanic voters to elect politicians who will enact more sweeping reforms to fix the U.S. immigration system. Arguing that no executive action on immigration could be as comprehensive as what Congress could do, he urged Hispanics at the black-tie dinner to go into their communities to ensure voters don't stay home.

"Yes we can - if we vote," he said, first in Spanish and then in English, in a twist on his 2008 campaign slogan.

The White House has been coy about what unilateral actions Mr. Obama and his administration are considering, and legal experts differ about just how far the president can go without Congress.

Immigration activists are calling for Mr. Obama to act aggressively to free a sizeable portion of the 11.5 million immigrants here illegally from fear of deportation.

Such a possibility has incensed Republicans, who say his willingness to ignore existing laws is the key reason they're reluctant to work with him to pass new ones.

"The president's promise isn't about making the best policy or enforcing the law - it's an admission that his pledge to not uphold the law in the future would be bad for his party now," Senate Minority Leader Mitch McConnell, R-Ky., said.

A supportive crowd offered the president a mostly warm reception, although he was briefly interrupted by a heckler who objected to deportations under the president's watch and was escorted out of the hall. Outside the convention center, a group of demonstrators gathered to protest Mr. Obama's delay.

And at the podium, he was gently nudged by Sen. Robert Menendez, D-N.J., who said Hispanics were looking to Mr. Obama "for big, bold, unapologetic" relief without delay.

"We need major reforms, we need them now," he said, "and Mr. President, we need your help."