The massive stimulus package boosted real GDP by up to 4.5 percent in the second quarter of 2010 and put up to 3.3 million people to work, the nonpartisan Congressional Budget Office said on Tuesday.

CBO's latest estimate indicates that the stimulus effort, which remains a political hot potato ahead of the November congressional elections, may have prevented the sluggish U.S. economy from contracting between April and June.

Economists surveyed by Reuters expect that revised numbers due out on Friday will show that the economy grew at an anemic 1.4 percent pace during that time period — less than the boost of at least 1.7 percent that the stimulus provided, according the CBO estimate.

The stimulus package, passed in March 2009 in the midst of the deepest recession since the 1930s, raised employment by between 1.4 million and 3.3 million jobs during that time period, CBO estimated.

CBO said the package, officially known as the American Recovery and Reinvestment Act, would cost $814 billion, down from its previous estimate of $862 billion.

With both the House of Representatives and the Senate up for grabs in November, Democrats hope voters will give them credit for breathing some life into the economy. Republicans, who almost universally opposed the stimulus, have criticized it as wasteful and ineffective.