WASHINGTON - On the eve of another fiscal showdown with congressional Republicans, President Obama is outright refusing to negotiate over an increase to the nation's debt limit, saying doing so would alter "the constitutional structure of this government entirely."

"What I haven't been willing to negotiate, and I will not negotiate, is on the debt ceiling," Obama told ABC News' George Stephanopoulos in an exclusive interview on "This Week."

"If we continue to set a precedent in which a president … is in a situation in which each time the United States is called upon to pay its bills, the other party can simply sit there and say, 'Well, we're not going to put - pay the bills unless you give us … what we want,' that changes the constitutional structure of this government entirely," Obama said.

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House Republicans, seeking to defund and delay implementation of the president's signature health care law, have sought to use the upcoming debt ceiling and government funding fights to extract concessions from the White House.

Obama says he is drawing a line in the sand.

"What has never happened in the past was the notion that in exchange for fulfilling the full faith and credit of the United States, that we are wiping away let's say major legislation like the health care bill," he told Stephanopoulos.

"Never in history have we used just making sure that the U.S. government is paying its bills as a lever to radically cut government at the kind of scale that they're talking about," he added.

The president said he believes there are areas of common ground between the parties on reforms to entitlement programs and the elimination of corporate tax loopholes. But, he said, an obstinate "faction" of the GOP is standing in the way of compromise.

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"All I can do when it comes to that group of members of Congress is to continue to talk to 'em and say, 'Let's put aside our differences. Let's stay focused on the American people,'" Obama said. "If we do that, we can get things done."

ABC News

In the past three weeks, Obama and his top aides have been in direct contact with more than 450 members of Congress on the situation in Syria, holding private dinners, conference calls, small group briefings and visits to the White House.

Obama's personal outreach to members on fiscal debates has been historically less robust. The White House cancellation of the annual congressional picnic this month will likely not help bolster positive relations heading into the fall.

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