If historical patterns are any indication, September could provide an attractive time to buy stocks ahead of the November mid-term elections.

Charts of the Standard & Poor's 500 index indicate that over the past several decades, in the second year of a presidential term, late summer weakness in stocks turns to an oversold or similar condition that attracts buyers in September or beyond, according to Concept Capital analysts in New York.

"July tends to be a good month, and August tends to be a volatile month and skewed to the downside. It's possible the market is tracing out the four-year presidential cycle where a better buying opportunity may present itself later on, perhaps in September," said John Kolovos, a technical analyst at Concept Capital.

The S&P 500 is now down 0.4 percent for the year-to-date, after gains of 6.9 percent in July, the best month for stocks in a year.

The index has been stuck in a trading range of about 1,070 to 1,130 recently, and Concept Capital analysts said August tends to be the weakest of summer months.

Over the last 20 years, the average monthly return for August has been just 0.1 percent, their research note said. In mid-term election years, August has fared worse, down on average 0.75 percent. In August 1990, it was down 9.3 percent, while in August 1998 it was down 14.6 percent, the note said.

The S&P ended 2009, the year President Barack Obama took office, with a gain of 23.4 percent. It started 2010 on a strong note, and was up more than 11 percent year-to-date in April before losing ground.

A spate of weak economic data has weighed on sentiment in recent weeks, and Friday's U.S. government jobs report showing private employers added fewer workers to their payrolls in July than expected gave investors further reason to sell.

"It points to continued expectations the economic slowdown we've seen will probably extend another two to three months, if not longer," said Fred Dickson, chief market strategist, The Davidson Cos. in Lake Oswego, Ore.

Those economic worries could make this mid-term election year behave differently than in the past.

But Kolovos said buying at the next oversold cycle makes sense from a technical standpoint.

The elections, which include voting for U.S. senators and representatives, state legislators and governors, occur in early November.

"A lot of people are looking at this point of the four-year cycle very closely," to see how strong the following year might be, Kolovos said.