WASHINGTON — Breakthrough coronavirus cases have emerged in multiple offices in Congress, including the speaker’s. The line for in-house testing snakes through a long corridor and into a visitors’ center atrium. The Capitol’s doctor has warned of the possible return of a mask mandate.

The delta variant has reached Capitol Hill, but a common enemy has only made recriminations and anger worse between the two political parties. Republicans, caught between a political base that is often resistant to vaccination and an imperative to save the lives of their voters, point their fingers at Democrats and blame them, without evidence, for covering up the virus’s origins.

Democrats fault Republicans who have done little to push back against vaccine skeptics in their ranks and even now are soft-pedaling their calls for people to take the shot.

“We’ve got people here who’ve refused to get vaccinated and are actually discouraging others to get vaccinated,” said Rep. Jim McGovern, D-Mass. “The Republican Party no longer lives in reality. It’s pathetic.”

For much of the vaccinated nation, the coronavirus resurgence is somewhere else. In states like Vermont, Hawaii and Massachusetts, where at least 84% of the adults have had one shot or two, surges in Alabama, Florida, Missouri and Arkansas are far, far away.

But the Capitol is one of the few places in America where red and blue mingle almost daily — and resentment is high.

“Congress is like a nationwide convention every single day,” said Rep. Jamie Raskin, D-Md., who has begun wearing a mask again, though he is fully vaccinated and 76.5% of adults in his state have received at least one shot. “There are people who have come from every corner, hamlet and precinct of the country. It’s a petri dish for the development of political ideas but also plagues.”

Republicans point out that the most recent high-profile carriers of the current plague were Democrats, Texas legislators who fled Austin to stop passage of a measure restricting voting. Six of them — all of whom said they were vaccinated — then tested positive and are suspected of infecting a senior aide to Speaker Nancy Pelosi. The aide, also vaccinated, is mildly symptomatic.

To be fair, the Texas legislators who made a show of defiance, then brought the virus, spread fears in both parties. Rep. Kim Schrier, D-Wash., a pediatrician, said those six breakthrough cases were a wake-up call in the Capitol community.

On the House floor, at least among Democrats, masks are going back on. Lawmakers are sending staff members for testing. Pelosi and some senators have told aides to work from home — just weeks after many of them returned to Capitol Hill.

And as lawmakers and aides look toward the fall, when school resumes before children younger than 12 are expected to have access to vaccines, their worries only worsen.

“If you’re in an area with a lot of unvaccinated people and you have unvaccinated kids, I would recommend you put your masks on again,” Schrier said. “If I had children under 12, I would be taking very big precautions right now.”

It is still unclear how many Republicans in the House and Senate are vaccinated, as many of them have refused to say one way or the other. Both Sens. Rand Paul, R-Ky., and Ron Johnson, R-Wis. — who has pushed fringe theories about the virus — have said they will not get the shot because they have already had COVID-19. (The Centers for Disease Control and Prevention recommends that people who have had the virus still be inoculated.)

Rep. Lauren Boebert, R-Colo., proudly proclaimed this month at the Conservative Political Action Conference in Dallas, “Don’t come knocking on my door with your ‘Fauci ouchie.’ You leave us ... alone.” She was referring to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, who has become a boogeyman for the right.

Congress’ attending physician, Dr. Brian Monahan, indicated that some remained unprotected when he pleaded this week with lawmakers in a memorandum: “The delta variant is a severe threat. I urge unvaccinated individuals to come for vaccination at any time.”

Leadership aides say the number of unvaccinated lawmakers is slowly dwindling. The No. 2 House Republican leader, Steve Scalise of Louisiana, got his first shot Sunday, a remarkable delay considering that one House Republican, Ron Wright of Texas, and one Republican member-elect from his home state, Luke Letlow, died of COVID-19.

But many Republicans will not divulge their vaccination status, even when pressed on whether they should be setting an example for their constituents.

“We believe in health privacy. The bottom line is, we believe it; it doesn’t stop at the COVID door,” Rep. Andy Harris, R-Md., a doctor, said Thursday. “It is every citizen’s right to choose to get a vaccine and then to choose not to reveal whether they’ve gotten a vaccine.”