Hopes for Covid-19 Vaccine Spark Partisan Fight Over Safety, Timing

WASHINGTON—A debate over the safety and timing of a possible Covid-19 vaccine has become a flashpoint in the presidential campaign and within the Trump administration, as well as in a key Senate race that could determine control of the chamber next year.

The fight also could have broad ramifications on the national discussion shaping people’s willingness to get a vaccine once it is available, with polls showing sharp differences between Democratic and Republican voters in their...