After an all too brief respite, the United States is again at a crossroads in the pandemic. The number of infections has ticked up — slowly at first, then swiftly — to 51,000 cases per day, on average, more than four times the rate a month ago. The country may again see overflowing hospitals, exhausted health care workers and thousands of needless deaths.

The more contagious Delta variant may be getting the blame, but fueling its rise is an older, more familiar foe: vaccine hesitancy and refusal, long pervasive in the United States. Were a wider swath of the population vaccinated, there would be no resurgence — of the Delta variant, or Alpha variant, or any other version of the coronavirus.

While mild breakthrough infections may be more common than once thought, the vaccines effectively prevent severe illness and death. Yet nearly half of the population remains unvaccinated and unprotected. About 30 percent of adults have not received even a single dose, and the percentage is much higher in some parts of the country.

America is one of the few countries with enough vaccines at its disposal to protect every resident — and yet it has highest rates of vaccine hesitance or refusal of any nation except Russia.

Public health experts have fruitlessly warned for months that the virus — any version of it — would resurge if the country did not vaccinate enough of the population quickly enough. Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, predicted in January that Florida might have a rough summer. Now one in five new infections nationwide is in Florida.

True, the speed and ferocity with which the Delta variant is tearing through Asia, Europe, Africa and now North America has taken many experts by surprise. It now accounts for about 83 percent of the infections in the United States.

But Delta is by no means the wickedest variant out there. Gamma and Lambda are waiting in the wings, and who knows what frightful versions are already flourishing undetected in the far corners of the world, perhaps even here in America.

Every infected person, anywhere in the world, offers the coronavirus another opportunity to morph into a new variant. The more infections there are globally, the more likely new variants will arise.

The United States will be vulnerable to every one of them until it can immunize millions of people who now refuse to get the vaccine, are still persuadable but hesitant, or have not yet gained access. The unvaccinated will set the country on fire over and over again.

And they will not be the only ones who are singed. Vaccinated people will be protected from severe illness and death, but there may be other consequences. Already in some communities, they are being asked to wear masks indoors. If the numbers continue to soar, the restrictions that divided the country before may return. Workplaces may need to close again, and schools, too.

And some number of vaccinated people will become infected. Breakthrough infections were expected to be vanishingly rare with the original virus, but recent data suggest they may be less so with the Delta variant. It is roughly twice as contagious as the original coronavirus, and some early evidence hints that people infected with the variant carry the virus in much higher amounts.

“The larger the force of infection that comes from the pandemic in unvaccinated populations, the more breakthrough infections there will be,” Dr. Hanage said.

Most breakthrough infections produce few to no symptoms, but some may prompt illness in vaccinated people serious enough to lay them up in bed, miss work — and put their children or older relatives at risk. Some cases may lead to long Covid, scientists now fear — a poorly defined syndrome in which symptoms seem to persist months.

This grim redux has a glaringly obvious solution: shots in arms. But short of a federal mandate — or a patchwork of mandates by municipalities, hospitals, colleges and businesses — it is hard to see how enough Americans will be immunized to form a buttress against the virus.

After a brisk vaccination campaign in the spring, the pace has slowed to about 537,000 doses per day, according to data gathered by The New York Times. Some responsibility for the lag lies with the frank refusal of conservative leaders — often Republicans — to champion the vaccines.

But misinformation, an epidemic all its own on social media, emanates from all parts the cultural spectrum, and there is no single reason so many Americans remain unvaccinated. It is a Hydra-headed problem.

Of the 39 percent of adults who are unvaccinated, about half say they are completely unwilling. But even within that group, some say they would comply if required to do so.

Some are hesitant and may come around with the right persuasion from people they trust, while still others plan to be inoculated but say they have just not had the chance.

Politics is a driver for only some of these people, noted Dr. Richard Besser, a former director of the Centers for Disease Control and Prevention. In New Jersey, where he lives, the rates vary drastically because of socioeconomic factors. In mostly white Princeton, 75 percent of adults are immunized, versus 45 percent in Trenton, just 14 miles away, which is heavily Black and Latino.

“Both are strong Democratic areas, so it’s really important to break things down and to address the issues that are impeding vaccination progress in each segment of the unvaccinated population,” Dr. Besser said.

Still, there is no doubt that the political divide is playing a role in rising infection rates. From the start, vaccinations in counties that voted for Donald J. Trump lagged those in counties that voted for Joseph R. Biden, and the gap has only widened — from two percentage points in April to nearly 12 points now, according to one recent poll by the Kaiser Family Foundation.

Nationwide, 86 percent of Democrats have had at least one shot, compared with 52 percent of Republicans, according to another poll. Even the national goal of having 70 percent of adults vaccinated by July 4 somehow became “Biden’s goal,” said Dr. Nahid Bhadelia, director of the Center for Emerging Infectious Diseases Policy and Research at Boston University.

“All of a sudden, even getting out of pandemic became a left versus right issue.”

Fewer than half of House Republicans are vaccinated as of May, compared with 100 percent of congressional Democrats. For months, some Republican lawmakers including Senators Ron Johnson of Wisconsin and Rand Paul of Kentucky, and conservative news commentators like Tucker Carlson, have voiced their skepticism of vaccines, loudly and insistently.

Lately, as infections rise in conservative precincts, a few Republican leaders have begun championing vaccination. Senator Mitch McConnell of Kentucky, the minority leader, who survived polio as a child, has worn masks and has urged that everyone be immunized. Senator Mitt Romney, Republican of Utah, said in an interview on Wednesday that “the politicization of vaccination is an outrage and frankly moronic.”

All of these leaders, and many more, will need to repeat vaccine affirmations often enough to persuade millions of people to overcome their hesitation. The Delta variant is thriving amid American discord. The vaccines are the remedy not just for this variant, but all those yet to come.