Six years of Donald J. Trump largely blocking out all other voices in his party have left Republicans without a credible messenger to push vaccines, even if they wanted to. Senator Mitch McConnell, the minority leader, may be using his campaign money to air pro-vaccine ads in his native Kentucky, but he is hardly a beloved figure within the party and is viewed by its base as just another member of the Washington establishment.

There are certainly other communities of vaccine resisters, including demographics of people who have historically been mistreated by the federal government (and also a small-but-vocal minority of professional athletes and Olympians), but it is Republicans and Republican-run states that have emerged as the biggest hurdle in America’s vaccination efforts.

With little ability to persuade the vaccine-hesitant and little help from the party he had pledged to work with, Mr. Biden and the federal government were left with a move he had resisted for weeks: make life more difficult for the unvaccinated, to try to force them to change their minds.