And Mr. Grassley finds himself on that road.

“It’s a volatile year, and Senator Grassley is in the middle of a constitutional crisis,” said Senator Richard J. Durbin, Democrat of Illinois. “I wouldn’t be surprised if across the country there is a new level of interest in House and Senate races. Remember what Trump said, ‘Delay delay delay,’ ” Mr. Durbin said.

Mr. Grassley has come under fire in Iowa for his refusal to hold hearings on a potential nominee for the Supreme Court after the death of Justice Antonin Scalia last month. The issue became more local for Mr. Grassley on Wednesday when it was revealed that the White House was considering Ms. Kelly as a potential replacement.

Then came word that Patty Judge, 72, a former Iowa lieutenant governor and state agriculture secretary, will seek the Democratic senate nomination to challenge Mr. Grassley.

For their part, many Republicans on Thursday, as they strolled into a lunch of various meats and chocolate-covered bacon sponsored by the other Republican senator from Iowa, Joni Ernst, declined to say whether they would prefer to have the Supreme Court nominee named by Mr. Trump or Mr. Obama.

“I wouldn’t want to rank it,” said Senator Jeff Flake of Arizona. Senator John McCain, the senior senator from Arizona, who earlier in the day criticized Mr. Trump’s foreign policy credentials, said he had “no idea” if Mr. Trump could make a suitable choice for the court. Nevada’s other senator, Dean Heller, a Republican, laughed at the question.

While Democrats did not directly link Mr. Trump’s inflammatory racial statements to the Supreme Court issue, they have made a concerted effort to tie the Republicans’ refusal to consider Mr. Obama’s nominee to Mr. Trump’s rise in the presidential race, and to frame the court fight as a battle over civil rights.

To emphasize the point, Democrats brought forth Representative John Lewis of Georgia, a hero of the civil rights movement, and others to highlight Supreme Court decisions like the one ending the Montgomery, Ala., bus boycott in 1956 and Brown v. Board of Education, which desegregated schools.