The rating agencies have put the U.S. on notice, as Forrest Gump might say, again: Come up with a credible plan to deal with the budget deficit or face yet another downgrade. And that warning has investors wondering what, if any, moves to make with their money should Uncle Sam's credit rating get dinged -- again.

We've been down this road before. A little less than a year ago, the rating agencies, after issuing a similar warning, did in fact downgrade the U.S. credit rating (in the case of Standard & Poor's, by one notch from triple-A to double-A), and nothing all that bad happened to most of our investments. Yes, there was a good deal of volatility last August. But, ultimately, with few exceptions, the sky didn't fall. Stocks, bonds and commodities didn't collapse.

There was no "rapid re-pricing" as some money managers said would happen when the credit agencies followed through on their warnings. "Medium- to long-term, it has not appeared to have any substantive impact, said Greg Gocek, a financial analyst and independent investor in the Chicago area.

Now, many experts don't expect history to repeat itself. Investment professionals and others think lawmakers will reach a temporary agreement on the debt limit and avert another downgrade.

4 reasons a downgrade won't happen

What's more, the probability of a major budget deal being reached in early 2013 is higher than usual for at least four reasons, according Jeffrey Kleintop, a financial analyst and the chief market strategist with LPL Financial.

One, the economic impact of the many scheduled tax increases and spending cuts is already set to begin in 2013, he said. The fiscal head wind comprising both tax increases and spending cuts under current policy totals more than $500 billion, or 3.5% of the gross domestic product. The 2013 budget changes, primarily consisting of tax increases, are already in the law and would need to be changed to mitigate or restructure them to be less of an economic drag. If not, a return to recession may be looming in 2013.

Two, the debt ceiling will be hit again in early 2013 and require legislative action to approve an increase.

Three, the rating agencies have warned that they will be watching in 2013 for the U.S. to take actions to return to a path of fiscal sustainability.

And four, President Barack Obama and a newly elected Congress will have maximum political capital to make it happen in early 2013.

"The most likely outcome is a fiscal tightening that exceeds 1% of GDP is likely next year," said Kleintop. In fact, he predicts a fiscal tightening of more than $200 billion -- with about half from tax increases -- totaling about 1.3% to 1.5% of GDP. From his perspective, the $200 billion is made up of the likely expiration of the payroll tax cut ($110 billion), a reduction in discretionary spending (of about $80 to $90 billion), and the imposition of the 3.8% surtax on investment income from high earners ($21 billion).

"This combination of about $100 billion in tax increases and $100 billion in spending cuts may be the sweet spot for markets," Kleintop said. "It is significant but not enough, by itself, to cause a recession; it may allay the immediate concerns of the rating agencies and avert a downgrade of our debt, and it may boost confidence that we can address our long-term fiscal imbalances and return to the path of fiscal sustainability. This is why the makeup of Congress is so important as Washington attempts to avert the budget bombshell from going off and taking the economy and markets with it."

Funds mentioned on the next page: iShares MSCI Emerging Markets Index (EEM), iShares FTSE/Xinhua China 25 Index (FXI) and iShares MSCI Brazil Index (EWZ).

More from MarketWatch: