General Motors Corp. was set to blanket the U.S. with incentives on several of its vehicles heading into the Memorial Day weekend, an attempt to keep demand from softening further in June.

Somewhat surprisingly, GM isn't the primary factor in the now anticipated flat — or declining — sales in May. Instead it's DaimlerChrysler Corp., which reportedly is expecting to report a significant 15% drop in U.S. deliveries. The automaker is ending second quarter incentives one month early in order to start more aggressive marketing offers June 1.

GM is adding another $500 in regional rebates, bringing the incentives on some vehicles to more than $2,000. The '00 Chevy Blazer now has a rebate of $2,500. Rebates also are heavy for the '00 Malibu ($1,500) and '00 Buick Century Special Edition ($2,250). Most Pontiac-GMC models also offer a separate $500 referral discount.

Analysts say the U.S. Federal Reserve's recent interest rate hikes likely are a key factor in May's unexpected sales slowdown.