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Four years after Barack Obama campaigned on a promise of "hope and change," he now is admitting that a president isn't able to change Washington "from the inside" -- an admission that rival Mitt Romney promptly blasted as "the white flag of surrender."

President Obama, at a forum Thursday hosted by the Spanish-language TV channel Univision, lamented the challenge of shaking up the status quo in the capital.

"The fact that we haven't been able to change the tone in Washington is disappointing," Obama said, in response to a question about his greatest failure. "The most important lesson I've learned is you can't change Washington from the inside. You can only change it from the outside."

He didn't explicitly state how he defines that distinction, but he suggested change "from the outside" means getting more ordinary Americans involved in the process.

"That’s how I got elected, and that’s how the big accomplishments like health care got done, was because we mobilized the American people to speak out," he said. "That’s how we were able to cut taxes for middle-class families.

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    Obama also pointed to his administration's failure to pass comprehensive immigration reform as one his biggest failure, but Romney's campaign pounced on the "change" quote.

    "He's asking for another four years while admitting he can't change Washington," senior Romney adviser Kevin Madden said.

    And Romney made it a key part of his speech Thursday at a rally in Sarasota, Fla.

    "The president today threw in the white flag of surrender again. He said he can’t change Washington from inside, he can only change it from outside," Romney said. "Well, we’re going to give him that chance in November. He’s going outside!

    "I can change Washington, I will change Washington. We’ll get the job done from the inside. Republicans and Democrats will come together. He can’t do it."