WASHINGTON (Reuters) -- The U.S. budget deficit this year could dip to as low as $300 billion, well below the White House's estimate, partly because the federal government was enjoying "robust growth" in revenues, the Congressional Budget Office said on Thursday.

In February, the White House estimated a possible fiscal 2006 budget deficit of $423 billion, a figure that political opponents said was likely inflated.

More recently, there have been government estimates of a budget deficit closer to $350 billion this year, down from the fiscal 2005 deficit of $318 billion.

The nonpartisan congressional budget analyst said that "robust growth in revenues" accounted for some of the improvement in the deficit picture.

The $300 billion estimate assumes that some version of an emergency spending bill for war and hurricane rebuilding will be enacted. President George W. Bush wants that measure to cost no more than $94.5 billion.

The Senate on Thursday passed a $108.9 billion bill that Bush has threatened to veto.

CBO's deficit estimate also assumed Congress would finish work this year on a $70 billion tax-cut package that includes tax breaks on investment income.

CBO said that during the first seven months of the fiscal year that began on Oct. 1, the federal government ran a budget deficit of $183 billion, $53 billion less than for the same period in fiscal 2005.