Iowa’s governor is not on the ballot next month. But her defiant attitude toward the advice of health experts on how to fight the coronavirus outbreak, as her state sees a grim tide of new cases and deaths, may be dragging down fellow Republicans who are running, including Mr. Trump and Senator Joni Ernst.

Ms. Reynolds, the first woman to lead Iowa, is an avatar of the president’s approach to the pandemic, refusing to issue mandates and flouting the guidance of infectious disease experts, who say that universal masking and social distancing are essential to limiting the virus’s spread. Defying that advice has eroded support for both Mr. Trump and Ms. Reynolds in Iowa, especially among voters over 65, normally a solid Republican constituency, according to public and private polls.

“Our older Iowans — many have not been able to leave their homes because they do not feel safe,” said Representative Cindy Axne, a first-term Democrat who represents Des Moines and southwest Iowa. “If you go into a grocery store, the large majority of people are not wearing masks.”…

A Monmouth University poll on Thursday showed Democrats are leading in three of Iowa’s four congressional races, with even the fourth, in deeply conservative Northwest Iowa, unexpectedly tight.