The Biden administration is running out of ideas for jumpstarting the pace of coronavirus vaccinations, raising the prospect that more than a quarter of American adults could still be vulnerable to the virus into the fall.

The federal immunization campaign has slammed into rising partisanship and deep resistance among the 91 million adults who remain unvaccinated, turning what was once an all-out sprint into a marathon with no clear end in sight.

Even as President Joe Biden vows a "neighborhood-by-neighborhood" vaccination effort, a half-dozen federal and state officials working on the Covid-19 response acknowledged in interviews that none of their outreach efforts are likely to supercharge vaccination rates — and that there are few remaining options to try.

The administration is now strategizing over how to manage a nation with 68 percent of the population at least partially vaccinated, where pockets of the country will be subjected to periodic outbreaks while the majority of Americans move on.

Officials foresee a grinding effort to convert people one shot at a time that could last for months, and that risks being complicated by growing Republican resistance to what was once a largely bipartisan bid to get the country immunized.

"We are under no illusions that each person in this stage will take longer to reach," said a senior administration official. "The first 180 million were much easier than the next 5 million."