If the government shutdown goes on as long as the last one 17 years ago, economic growth in the final three months of the year could be nearly cut in half, Beth Ann Bovino, chief U.S. economist at Standard & Poor's, told CNBC on Monday, as the federal closure entered its seventh day.
"The longer it goes, the more it's going to hurt. If you're saying something closer to the 1995-1996 shutdown of about a little less than a month, you're looking at about 1.2 percent of growth taken off of the fourth quarter," she calculated in a "Squawk Box" interview. "That's pretty significant."
S&P has estimated that for every week of the government shutdown, there would be a 0.3 percent drag on gross domestic product.
(Read more: Boehner takes a hardline in call for concessions)
Before the shutdown, Bovino had expected a relatively rosy fourth-quarter annual GDP growth rate of 3.0 percent. Under her worst-case scenario now, GDP could end up only increasing 1.8 percent in the last quarter of 2013.
"Where do you see that? IRS, no tax refunds go out to the people who need it. All those people on furlough? Well, they're not getting checks and eventually that's going to really hit their spending," she argued. "Then you have loans, [government] loans won't indeed be available for home loans and small-business loans. Then finally for economists like me and the Fed, you don't have any data to analyze what's going on in the world."
If the shutdown lasts four weeks and fourth-quarter GDP is as bad as Bovino predicts it could be, she said her full-year growth economic estimates for 2013 would likely drop from 1.7 percent to 1.5 percent.
Bovino also pointed out that growth has already been pretty slow lately. From the fourth quarter of 2012 through the second quarter of 2013, it's only averaging 1.25 percent, she added.
In addition to the debate over funding the government, Republican leaders and President Barack Obama need to come to agreement on increasing the debt ceiling before the Oct. 17 deadline.
On Friday, Standard & Poor's lead analyst on U.S. sovereign ratings, Marie Cavanaugh, told "Squawk Box" that the borrowing limit would be raised on time, and all this budget wrangling is "unlikely to change" S&P's rating on U.S. debt.
(Read more: US debt downgrade unlikely: S&P analyst)
Similar brinkmanship in the summer of 2011 over the debt ceiling, which was eventually increased then, resulted in the U.S. losing its Standard & Poor's sterling AAA credit rating. S&P's current AA-plus rating has a stable outlook.
—By CNBC's Matthew J. Belvedere. Follow him on Twitter @Matt_SquawkCNBC.