President Joe Biden speaks during a news conference in Hiroshima, Japan, on May 21 following the G7 Summit. (Susan Walsh/AP)

Hiroshima, Japan (CNN) — President Joe Biden issued a stark warning Sunday that congressional Republicans could use a national default to damage him politically and acknowledged time had run out to use potential unilateral actions to raise the federal borrowing limit, a sharp shift in tone days before the deadline to reach an agreement.

Characterizing GOP proposals as “extreme” and warning they couldn’t gain sufficient support in Congress, Biden said he wasn’t able to promise fellow world leaders gathered in Japan for Group of Seven talks the US would not default.

“I can’t guarantee that they will not force a default by doing something outrageous,” he said.

Biden’s remarks, delivered before he left for Washington, were the latest indication that talks between the White House and congressional Republicans remain far apart.

He was expected to speak by phone with House Speaker Kevin McCarthy after taking off aboard Air Force One, though it wasn’t clear whether that conversation would break the logjam.

“My guess is he’s going to want to deal directly with me in making sure we’re all on the same page,” he said.

Republicans have been seeking spending cuts in the federal budget in exchange for their support to raise the nation’s borrowing limit. On Sunday, Biden acknowledged “significant” disagreement with Republicans in some areas, insisting that while he’s willing to cut spending, tax “revenue is not off the table” as part of the deal.

McCarthy, in an interview later Sunday with Fox News, disagreed with that characterization, saying Biden previously told him that tax increases were “off the table” and that he wouldn’t agree to them.

“He’s now bringing something to the table that everyone said was off the table,” the California Republican said. “It seems as though he wants to fault more than he wants a deal.”

At his news conference, Biden said that much of what Republicans have proposed “is simply, quite frankly, unacceptable.”

“It’s time for Republicans to accept that there’s no bipartisan deal to be made solely, solely on their partisan terms. … They have to move, as well,” the president said.

Pressed on whether he would be to blame for a default scenario, Biden said that based on what he’s offered, he should be blameless but conceded that “no one will be blameless” as he suggested some of his political rivals could be encouraging a default to sabotage his reelection efforts.

“I think there are some MAGA Republicans in the House who know the damage it would do to the economy, and because I am president, and a president is responsible for everything, Biden would take the blame and that’s the one way to make sure Biden’s not reelected,” he said.

McCarthy, in turn, blamed what he called the “socialist wing of the Democratic Party” for driving Biden’s goals in the negotiations.

“The president keeps changing positions every time Bernie Sanders has a press conference. He gets reactive and he shifts,” the speaker said as he arrived at the US Capitol in Washington on Sunday.

Meanwhile, Biden’s top national security aide told CNN that the stalled debt ceiling and budget negotiations have not undercut American leadership abroad or undermined the G7 summit as it came to a close Sunday.

“When you look at the totality of the last three days, it’s actually a reflection of and an exclamation point on the way in which President Biden has led on the world stage. People understand democracies, and they understand that there are moments in domestic politics when you have got to look at the home front,” national security adviser Jake Sullivan told CNN’s Jake Tapper on “State of the Union.”

Biden outlines shortcomings of 14th Amendment argument

Biden in his news conference addressed the possibility of using the 14th Amendment to continue US government borrowing in the absence of a deal, suggesting he has the power but not the time to utilize the unilateral action.

“I think we have the authority. The question is, could it be done and invoked in time that it could not – would not be appealed?” Biden asked, calling the question of whether an appeal could be solved before the default deadline “unresolved.”

Pressed by CNN’s Phil Mattingly to clarify whether he thought he could invoke the 14th Amendment as a serious and tangible option, the president made clear that maneuver would not be successful given the short window remaining.

“We have not come up with unilateral action that could succeed in a matter of two weeks or three weeks. That’s the issue. So it’s up to lawmakers. But my hope and intention is to resolve this problem,” he said.

Republican Sen. Bill Cassidy of Louisiana said Sunday a potential invocation of the 14th Amendment would be a “dodge.”

“The president needs to show leadership. ‘OK, House Republicans, American people, you’re concerned about spending, I will meet you there. As opposed to finding a dodge that tries to work its way around,” Cassidy said.

Treasury Secretary Janet Yellen has warned the US could default on its debts as soon as June 1.

Talks at a standoff in Washington

On Saturday afternoon, McCarthy had said negotiators wouldn’t be able to resume talks with the administration until Biden was back in Washington.

“Unfortunately, the White House moved backwards,” he said. “I don’t think we’re going to be able to move forward until the president can get back.”

Biden had originally planned to stop in Australia and Papua New Guinea after the G7 summit in Hiroshima, he canceled those portions of the trip amid the debt ceiling talks.

Biden asked his team to coordinate with the speaker to arrange the conversation on Sunday morning Eastern Time, which would be the two men’s first conversation since debt talks appeared to stall amid disputes over spending limits.

Republican Rep. Patrick McHenry of North Carolina, who has served as one of the chief negotiators during the debt ceiling talks, said Sunday that he was “not at all” optimistic that a deal can come together.

“I’ve been pessimistic for a while, and something needs to change,” he told CNN.

Earlier Saturday, Republican Rep. Dusty Johnson of South Dakota confirmed that the White House had made an offer seeking to cap future spending at current levels, which Johnson called “unreasonable.”

Johnson, a McCarthy ally and chair of the centrist Main Street Caucus, is one of several key players who has been getting briefed by Republican negotiators on the talks.

“Negotiations did not go well today,” Johnson said. “The paper that the White House provided was a major step backward. And it undermined all the progress that was made Wednesday and Thursday. … It has endangered negotiations.”

Johnson warned, “We are at real risk of default.”