Asked if he thought the deficit for fiscal year 2007, which began October 1, would come in below the forecast of $339 billion the White House gave in its mid-year budget snapshot in July, Portman replied, "Yes, I do."
Iraq war spending hit an all-time high of $120 billion in fiscal year 2006 that ended on September 30.
Some media reports have said the war costs for 2007 could total around $170 billion. But Portman declined to give a precise figure.
The White House is scheduled to unveil its 2008 spending blueprint in early February.
Along with the spending plan, the administration will offer a fresh request for money for the war and will provide updated forecasts for the budget deficit.
Portman declined to comment on the costs that would be involved if 20,000 additional troops are sent to Iraq in the near future.
Media reports have said that option is being considered by George Bush, the US president, who will unveil an new strategy for the war early in the new year.
Democrats urge accountability
Nearly four years into the Iraq war, Bush has maintained a practice of using emergency-spending bills to finance the costs of the war.
But the Iraq Study Group, a high-level panel that recommended a change in course for the war, also suggested that the Iraq war costs be included in the annual budget process to make them more transparent.
Democrats, who are set to take over Congress in January, have also pressed the administration to put the war costs into the regular budget.
The White House has resisted doing so, saying the costs are difficult to predict.