* ISM manufacturing, construction spending data on tap
* Moody's changes EU rating outlook to negative
* Spain, Italy yields fall as ECB speculation grows
* S&P down 0.3 pt, Dow up 9 pts, Nasdaq off 2.25 pts
By Angela Moon
NEW YORK, Sept 4 (Reuters) - Wall Street was set for a flat open on Tuesday as investors returned after a long weekend, with focus on U.S. manufacturing data and an upcoming meeting of European Central Bank policymakers.
Moody's Investors Service has changed its outlook on the Aaa rating of the European Union to negative, warning it might downgrade the bloc if it decides to cut the ratings on Germany, France, UK and Netherlands.
The Institute for Supply Management releases its August manufacturing index at 10:00 a.m. ET (1400 GMT). Economists expect a reading of 50.0, versus 49.8 in July.
The Commerce Department releases July construction spending numbers at 10:00 a.m. ET (1400 GMT). Economists forecast a rise of 0.4 percent, a repeat of the June increase.
S&P 500 futures fell 0.3 point and were in line with fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures gained 9 points, while Nasdaq 100 futures lost 2.25 points.
"Expectations for QE3 seem to suggest it will be happening sooner, rather than later. However, Wall Street is taking a guarded stance toward the markets ahead of this morning's manufacturing report," said senior options strategist Tony Venosa at Schaeffer's Investment Research.
In Europe, Spanish and Italian government bond yields fell after the head of the European Central Bank hinted at the scope of a much-anticipated bond buying program.
The ECB is expected to unveil its debt-purchasing plan to tackle the region's debt crisis at a policy meeting on Thursday, where it may also cut rates as the 17-nation euro area heads toward a recession.
European shares were lower, however, having already risen strongly on hopes that central bank action could start the fightback against the euro zone's chronic debt problems.
But the "positive mood remains intact on account of still high expectations that the ECB will unveil further detail of the timetable surrounding purchases of short-dated government bonds in secondary markets in response to official requests going forward," said Andrew Wilkinson, chief economic strategist at Miller Tabak & Co. LLC.
On Monday, China's official purchasing managers' index fell below the 50 level that demarcates expansion from contraction for the first time since November 2011, while a similar survey from HSBC showed activity shrinking at its fastest pace since March 2009.
Xerox Corp and a wholly-owned unit that contracted with Texas to process dental claim forms are being investigated to see if they helped allow dentists to submit higher-than-required bills under the state's Medicaid system, the Wall Street Journal reported.
U.S. Secretary of State Hillary Clinton has pledged to take a strong message to Beijing this week on the need to calm regional tensions over maritime disputes that have raised broader fears of military friction between the two major Pacific powers.
U.S. stocks rose on Friday after Federal Reserve Chairman Ben Bernanke, expressing "grave concern" for the stagnating U.S. job market, said the central bank was prepared to take further steps to strengthen the economy if necessary.