TIPS = Treasury Inflation Protected Securities.
Inflation could come quickly if the U.S. starts printing money to pay debt. I'm thinking about investing in these to protect my portfolio.
How does the U.S. credit rating affect these securities?
Lowering of the US credit rating would affect all US bonds. Some institutional investments are required to invest in securities with a certain credit rating (i.e. money markets and some low risk mutual funds). If the credit rating is lowered these institutions would be required to dump their US bond holdings. This could have a serious affect on bond prices. The lower bond prices would drive up yields.
If the US credit rating was lowered after you purchased TIPS then the price you could sell your TIPS for would most probably be lower then what you bought them. You would lose money.
All US bonds, including TIPS, would be affected by a lower credit rating since the credit rating is suppose to indicate the borrower's ability to repay the debt. This is independent of inflation. TIPS provide no additional benefit over regular bonds in regard to credit rating.