Washington — Sen. Barbara A. Mikulski of Maryland will “pinch-hit” for Sen. Edward M. Kennedy of Massachusetts in negotiations over the reauthorization of the Higher Education Act while Mr. Kennedy is being treated for a brain tumor, Congressional Quarterly reported today.

Mr. Kennedy, who learned last month that he had a type of tumor called a malignant glioma after suffering a seizure on May 17, is undergoing surgery today at the Duke University Medical Center. He is expected to remain at the hospital for a week and then head home for chemotherapy and radiation treatment.

Assuming the treatment is effective, it is unclear when Mr. Kennedy would return to the Senate, though he could be out for several weeks.

In his absence, Ms. Mikulski will oversee member-to-member negotiations over the most contentious provisions of the bill, which would set federal higher-education policy for at least the next five years. The bill is now in pre-conference, where aides in the House and Senate are hashing out a compromise in advance of a formal member conference.

Ms. Mikulski is the third-most-senior Democrat on the Senate education panel after Mr. Kennedy, the chairman. The two more-senior Democrats, Christopher J. Dodd of Connecticut and Tom Harkin of Iowa, already lead other committees.

In a written statement, Mr. Kennedy said he looked forward to returning to the Senate and “to doing everything I can to help elect Barack Obama as our next president.” —Kelly Field