Think September’s market action is bad? Buckle up, then. Goldman Sachs tactical specialist Scott Rubner noted Wednesday that the second half of September has been the worst two-week period of the year for the S & P 500 going back to 1950. During that period, the broad market index tends to lose nearly 0.5% on a median basis. It gets worse. Going back to 1928, the median S & P 500 return is negative in the final 10 of 11 days to end the month, Rubner added. The stock market is already off to a rough start for September. The S & P 500 is down 2.3% through the first two sessions of the month. That includes a 2.1% drop on Tuesday — after two new data reports renewed fears over the state of the U.S. economy. “I am bearish on U.S. equities starting on September 16th, however we are starting to see this thesis start to get pre-traded by market participants as we enter September. We are seeing clients get ahead of negative market technicals sooner rather than waiting for mid-month,” Rubner wrote. “A market correction may start to get traction if payrolls are weak on Friday.” Economists polled by Dow Jones expect the economy added 161,000 jobs in August. To be sure, another employment report posted Thursday showed a much bleaker picture. ADP said private payrolls grew by 99,000 last month — well below a forecast of 140,000. Elsewhere on Wall Street this morning, Wolfe Research downgraded General Motors to peer perform. “Despite Mgmt’s targets for reaching positive margins next year, investors remain skeptical given soft demand trends and high EV-structural costs,” analyst Emmanuel Rosner said.