A surprisingly broad section of the Republican Party is convinced that a threat once taken as economic fact may not exist — or at least may not be so serious. Some question the Treasury's drop-dead deadline of Oct. 17. Some government services might have to be curtailed, they concede. "But I think the real date, candidly, the date that's highly problematic for our nation, is Nov. 1," said Senator Bob Corker, Republican of Tennessee.
Others say there is no deadline at all — that daily tax receipts would be more than enough to pay off Treasury bonds as they come due.
"It really is irresponsible of the president to try to scare the markets," said Senator Rand Paul, Republican of Kentucky. "If you don't raise your debt ceiling, all you're saying is, 'We're going to be balancing our budget.' So if you put it in those terms, all these scary terms of, 'Oh my goodness, the world's going to end' — if we balance the budget, the world's going to end? Why don't we spend what comes in?"
"If you propose it that way," he said of not raising the debt limit, "the American public will say that sounds like a pretty reasonable idea."
In a news conference, Mr. Obama said repeatedly that those who doubted the repercussions of a default were making a huge mistake.
"When I hear people trying to downplay the consequences of that, I think that's really irresponsible, and I'm happy to talk to any of them individually and walk them through exactly why it's irresponsible," he said. "And it's particularly funny coming from Republicans who claim to be champions of business. There's no business person out here who thinks this wouldn't be a big deal, not one. You go to anywhere from Wall Street to Main Street, and you ask a C.E.O. of a company or ask a small-business person whether it'd be a big deal if the United States government isn't paying its bills on time. They'll tell you it's a big deal. It would hurt."
(Read more: Shutdown is shaking confidence: US Commerce Secretary)
The turmoil created by the partial shutdown of the federal government has already sent investors fleeing from stocks to the safe harbor of Treasury bonds, long considered the safest investment on earth because the full faith and credit of the United States government has never been questioned. If that safe harbor is undermined, most economists have said loudly and repeatedly, the impact could be catastrophic.
The U.S. Chamber of Commerce and the National Association of Manufacturers, both bastions of Republican support, sent letters to Congress on Tuesday urging action on the debt ceiling.
"Our nation has never defaulted in the past, and failing to raise the debt limit in a timely fashion will seriously disrupt our fragile economy and have a ripple effect throughout the world," wrote Jay Timmons, the president of the manufacturers' group.
Some Republicans trust such warnings.
"Unlike some of my colleagues, I've been told by too many people in the financial business that there will be reactions in the market," said Senator John McCain, Republican of Arizona. "Of course I'm worried."
But the voices of denial are loud and persistent, with some Republicans saying that the fallout from the continuing shutdown and the automatic, across-the-board budget cuts known as sequestration has been less severe than predicted.
Mr. Paul acknowledged that some economists disagreed with him, but said others agreed. Peter Morici, a conservative economist and a frequent guest on Fox Business, dashed off a column on Tuesday in which he argued that "House Republicans, by refusing to raise the debt ceiling until they obtain budget reforms, may be the country's last hope to avoid a financial ruin."
Congressional Republicans have varied arguments. To Representative Paul Broun, Republican of Georgia and a candidate for the Senate, it is a question of ranking the evils.
"There are a lot of things that are going to affect our economy," he said. "The greatest threat right now is Obamacare. It's already destroyed jobs, it's already destroyed our economy, and if it stays in place as it is now, it's going to destroy America."
(Read more: Boehner says Obama's position is 'unsustainable')
Representative Ted Yoho, a freshman Florida Republican who had no experience in elective office before this year, said the largest economy on earth should learn from his large-animal veterinary practice.
"Everybody talks about how destabilizing doing this will be on the markets," he said. "And you'll see that initially, but heck, I've seen that in my business. When you go through that, and you address the problem and you address your creditors and say, 'Listen, we're going to pay you. We're just not going to pay you today, but we're going to pay you with interest, and we will pay everybody that's due money' — if you did that, the world would say America is finally addressing their problem."
Representative Justin Amash, Republican of Michigan and a leader of the House's libertarian wing, said: "There's no way to default on Oct. 17. We will have enough money to make interest payments. The issue is, how do we restructure our government so we don't have to keep hitting the debt ceiling?"
And while Representative Trent Franks, Republican of Arizona, conceded that a government that could no longer borrow money would have to curtail some of its contracting, he said Democrats should not get carried away.
"It's like everything else here," he said. "People on both sides of every argument seem to employ hyperbole when they could just state the truth and it would still be of significant consequence."
—By Jonathan Weisman of The New York Times. Ashley Parker contributed reporting.