In December, Carat projected that U.S. ad spending would increase 4.1% this year over last year.
In February, it lowered its projection to 3.8% growth, citing weakening economic conditions.
Ad spending this year will be driven by the 2008 Beijing Olympics and the presidential election, Carat said in the report.
Next year, ad spending growth will improve slightly, to just over 3% growth, Carat projected.
A significant rebound in ad spending is not expected until 2010, according to the report.