Several states and municipalities have tried to outlaw flavored e-cigarettes and are now facing court challenges from the vaping industry. On Tuesday, the American Medical Association called for a far-reaching ban on all vaping products, as two more states sued Juul, the nation’s largest seller of e-cigarettes, over its marketing practices.

The questions about e-cigarettes, by members of the Senate’s Committee on Health, Education Labor and Pensions, highlighted how carefully Dr. Hahn would need to navigate a range of sensitive issues if he were confirmed to lead the F. D.A., a sprawling agency with oversight of everything from tobacco and e-cigarettes to food safety and the approval of drugs and medical devices.

At the hearing, Senator Mitt Romney, Republican of Utah, described the questions about e-cigarettes as a “canary in the coal mine” that would demonstrate how Dr. Hahn would respond to political pressure and corporate influence . “How you will deal with this issue is a pretty good test case for how you will deal with these issues on an ongoing basis,” he said.

Dr. Hahn sought to portray himself as a committed cancer researcher and medical doctor who would bring the values of those professions to his tenure as F.D.A. commissioner. “Patients need to come first, and the decisions that we make need to be guided by science and data, congruent with the law,” he said.