Laurence H. Tribe, the highly regarded liberal scholar of constitutional law, still speaks of President Obama as a proud teacher would of a star student. “He was one of the most amazing research assistants I’ve ever had,” Mr. Tribe said in a recent interview. Mr. Obama worked for him at Harvard Law School, where Mr. Tribe has taught for four decades.

Mr. Tribe went on to serve in the Justice Department during Mr. Obama’s first term and has argued in favor of the legal standing of Mr. Obama’s signature health care law and executive orders on immigration.

Which is why so many in the Obama administration and at Harvard are bewildered and angry that Mr. Tribe, who argued on behalf of Al Gore in the 2000 Bush v. Gore Supreme Court case, has emerged as the leading legal opponent of Mr. Obama’s ambitious efforts to fight global warming.