A Senate hearing on the federal government’s Covid response briefly turned into a fierce back-and-forth between Senator Rand Paul of Kentucky and Dr. Anthony Fauci on Tuesday, the latest in a series of heated exchanges between the two men stretching back to the early days of the pandemic. Mr. Paul accused Dr. Fauci of plotting with colleagues to undermine scientists with opposing views on the virus, something Dr. Fauci denied doing. The senator cited an email exchange between Dr. Fauci and Francis Collins, a former director of the National Institutes…