WASHINGTON (AP) — President Barack Obama's first budget is based on economic assumptions that are significantly higher than the forecasts of many private economists, which could make it harder to reach his goal of cutting the deficit in half by the end of his first term.

The administration insisted that it wasn't relying on overly optimistic assumptions to make the spending and deficit figures look better, but private economists said the economic predictions underlying the budget Obama released yesterday were far higher than their own forecasts.

"These numbers are a lot more optimistic than what I am forecasting both for this year and in the long run," said David Wyss, chief economist at Standard & Poor's in New York.

The administration's strong growth numbers would make it easier to achieve Obama's goal of cutting the inherited deficit in half by 2013. The administration predicted that the overall economy, as measured by the gross domestic product, will shrink by 1.2 percent this year but will grow by a solid 3.2 percent in 2010. That growth would be followed by even stronger increases of four percent in 2011, 4.6 percent in 2012 and 4.2 percent in 2013.

By contrast, the consensus of forecasters surveyed by Blue Chip Economic Indicators in February predicted that the GDP will fall by a larger 1.9 percent this year and then increase at weaker rates of 2.1 percent in 2010, 2.9 percent in 2011 and 2012 and 2.8 percent in 2013. Nariman Behravesh, chief economist at IHS Global Insight, a major private forecasting firm, called the administration's forecasts "way too optimistic" and said it could represent a return to the overly optimistic forecasts of previous administrations confronted by surging budget deficits. Behravesh said his own forecast was that GDP was likely to post a bigger decline this year of possibly as much as three percent and then weaker growth of around two percent in 2010 and 2.5 percent in 2011.

That conforms with expectations of many other private analysts who believe that the current recession and rebound will be more U-shaped than V-shaped, reflecting a downturn that will last for a period with only a gradual recovery.

Christina Romer, the head of the president's Council of Economic Advisers, defended the administration's stronger GDP forecast, contending that in previous severe recessions, the pattern often showed a stronger rebound once the downturn is over. She cited the Great Depression as one such episode when the economy rebounded by strong rates after years of sizable declines.

She also suggested that many private forecasters may not be adequately taking account of the size of the government support that has been put forward, including the $787 billion economic stimulus bill, the $700 billion financial rescue package which the new budget suggests may be doubled if necessary and the administration's mortgage mitigation effort.

"If there is ever a time when we think policy is going to contribute ... now is the time," she said.

But private forecasters said even taking into account the government's efforts, they still believed the recovery from the current downturn was likely to be more modest than the administration is predicting, given the significant headwinds facing the economy, including the worst financial crisis in seven decades. They predicted a U-shaped recovery instead of a V-shaped recovery.

Mark Zandi, chief economist at Moody's Economy.com, said he believed the extent of the downturn will be more severe than the administration's forecast for this year and that this will prompt even larger policy responses on the part of the government, including increased help for homeowners facing foreclosure.