More than 75 Nobel Prize winners have signed a letter urging senators not to confirm Robert F. Kennedy Jr., President-elect Donald J. Trump’s pick to lead the Department of Health and Human Services. The letter is the first time in recent memory that Nobel laureates have banded together against a Cabinet choice. The group tries to stay out of politics whenever possible. But the confirmation of Mr. Kennedy, a staunch critic of mainstream medicine who has been hostile to the scientists and agencies he would oversee, is a threat that the Nobel laureates could not ignore.