WASHINGTON  As the Obama administration presses ahead with the health care law, officials are bracing for the possibility that a federal judge in Virginia will soon reject its central provision as unconstitutional and, in the worst case for the White House, halt its enforcement until higher courts can rule.

The judge, Henry E. Hudson of Federal District Court in Richmond, has promised to rule by the end of the year on the constitutionality of the law’s requirement that most Americans obtain insurance, which takes effect in 2014.

Although administration officials remain confident that it is constitutionally valid to compel people to obtain health insurance, they also acknowledge that Judge Hudson’s preliminary opinions and comments could presage the first ruling against the law.

“He’s asked a number of questions that express skepticism,” said one administration official who is examining whether a ruling against part of the law would undermine other provisions. “We have been trying to think through that set of questions,” said the official, who insisted on anonymity because he was not authorized to discuss the case freely.