The tired Wall Street cliché “sell in May and go away” is looking pretty smart right now.

The U.S. stock market has run into a renewed wave of selling that is threatening to send major indexes through lows set in March that many had thought would turn out to be the bottom for 2008.

Near-record oil prices and further turmoil in the financial services industry combined to send the Dow Jones industrials down 220 points to 11,843 Friday. That left it down 3.8% for the week and 6.3% since the end of May.

Even more troubling is that the Dow now sits just 0.9% above its 2008 low of 11,740 set in March. The broader Standard & Poor’s 500 index is doing a bit better, still 3.5% above its March low.

With stocks marching lower, some on Wall Street are worrying what will happen if the selling continues and the indexes break through those psychological support levels. “The market lows are definitely in jeopardy,” says Chuck Stutenroth, managing director of ZAR Fund Group. If the Dow breaks its low, he thinks a further 15% drop could follow.

Investors who thought they’d seen the worst from stocks in March are having second thoughts now due to:

•Crude and credit. Skyrocketing oil prices have made an already weak economy even more precarious. While the price of crude was rising in 2007, the breathtaking 40% jump this year caught investors off guard.

Meanwhile, problems with the credit market continue. Merrill Lynch on Friday cut its earnings estimates for 12 banks by 22% in 2008. “The risks that face the market are too significant,” says Hugh Johnson, chief investment strategist at Johnson Illington Advisors.

•End of interest rate cuts. Crude and credit have been a problem for months, but at least short-term interest rates were falling. Starting in September, the Federal Reserve took the federal funds rate from 5.25% to the current target of 2.0%. Now the Fed is signaling rate cuts are over, Stutenroth says.

•Worsening signs from major U.S. businesses. Ford Motor on Friday said it will lose more money in 2008 and likely won’t break even in 2009 as it had forecast a month ago. Major debt-rating agencies in turn lowered their views on the Big Three automakers.

With bad news cropping up daily, investors would rather sell and “protect what they’ve got,” especially with the end of the quarter coming, says Oliver Wiener, trader at BTIG.

Ryan Detrick of Schaeffer’s Investment Research, though, says rising angst shows stocks are near a bottom. “Bottoms take time. They frustrate people to no end,” says Detrick, who thinks the March lows will hold. “Hopefully, this is just one more good shakeout.”