Senate Democrats lined up alongside Sen. Josh Hawley (Mo.), one of their least-favorite Republicans, to support a measure urging the Biden administration to declassify intelligence on whether the novel coronavirus originated in a Chinese lab. A Democratic-led House subcommittee is pledging an investigation into the virus’s origins, including the lab’s safety record.

And President Biden, in an unusual public statement, directed U.S. intelligence agencies to “redouble their efforts” to determine the cause of the pandemic, suggesting that while the virus could have jumped from animals to humans, it also could have escaped from the lab.

The White House was told a large amount of information remained to be examined that could shed light on the origins of the pandemic, according to a senior administration official.

The rapid developments mark a new effort by Democrats to show they are pushing to figure out how the pandemic started and, in the process, considering a theory that some initially attributed to conspiracy theorists: that the pandemic that has cost about 3.5 million lives worldwide stemmed from human error at the Wuhan Institute of Virology.

That thesis is far from conclusive; no significant new evidence has emerged to support it, and the pandemic’s origins may never be definitively known. Many still believe the virus jumped naturally from animals to humans. But some scientists who dismissed the theory early on have begun reassessing their views, and new evaluations have been aired in a recent piece in the Bulletin of the Atomic Scientists.

Republicans, saying they feel vindicated because some pointed to the lab early on, have been pushing the lab-leak theory more aggressively at congressional hearings and in conservative media outlets. And Democrats say the departure of former president Donald Trump, who often talked about the pandemic in racially charged terms, makes it easier to consider the theory without potentially offensive undertones.

The shifting terrain highlights how much of the early debate on the virus’s origins was colored by America’s tribal politics, as Trump and his supporters insisted on China’s responsibility and many Democrats dismissed the idea out of hand — when the origins of the virus were in fact wrapped in uncertainty.

The polarization, which left many feeling they had to embrace one theory or the other, was exacerbated by the tendency of some on the right to conflate the lab-leak theory with more easily dismissible ideas such as the notion that the coronavirus was part of a Chinese biological weapon.

“Like everything else, it became politicized very early on,” said Rep. Jamie B. Raskin (D-Md.), a member of the House select subcommittee on the coronavirus crisis.

Biden has enjoyed high approval ratings, at least among Democrats and independents, for his handling of the pandemic. He passed a $1.9 trillion stimulus package, ramped up a vaccination program that began under the Trump administration, and has begun steering the country toward normalcy in the aftermath of nearly 600,000 American deaths.

During much of this effort, Democrats have focused less — publicly at least — on the need to determine the origin of the pandemic that engulfed the world a year ago. That is now prompting a Republican effort to reclaim the “follow the science” mantra that Democrats used effectively in 2020 to position themselves as the party better equipped to end the pandemic.

Republicans are also seeking to use the episode to sow doubt about Biden’s ability to confront China, with some saying Biden’s ostensible reluctance to focus on the Wuhan lab shows he is soft on the rising superpower.

After Biden announced Wednesday he had given intelligence officials 90 days to come up with a clearer picture of the virus’s origins, Republicans wasted little time claiming vindication.

“The only reason that Biden’s doing this is it’s becoming untenable not to look into whether or not the virus originated in a Chinese lab,” Sen. Lindsey O. Graham (R-S.C.) said in a statement.

Democrats dismiss the GOP statements as posturing, noting that Republicans did little to challenge some of Trump’s more far-fetched theories on the virus, such as the idea that injecting disinfectant could cure it or his efforts to ridicule mask-wearing.

Some Democratic lawmakers also said they had never ruled out the Wuhan lab theory and that they have simply become more receptive to it as scientists and epidemiologists have done the same.

“The researchers themselves were skeptical about the possibility of the lab,” said Rep. Diana DeGette (D-Colo.), who chairs a powerful oversight panel on the House Energy and Commerce Committee.

DeGette said that in recent days she has been in talks with the top Republican on her subcommittee to determine how to best investigate the origins of the virus. She said her panel so far focused on emergency matters such as getting Americans vaccinated.

“We haven’t really had the luxury to sit back and say, ‘Okay, now what happened in that lab?’ ” DeGette said. “But I do think we need to get to that issue.”

Some Democrats argued that Republicans’ renewed emphasis on the lab theory was an effort to change the focus from the Trump administration’s fumbling of the pandemic.

“Democrats have been interested from the beginning,” Raskin said. “But we also recognized the way in which that question could be used as a distraction from the Trump administration’s own miserable failures in addressing the virus.”

Biden’s decision to roil the waters this week by publicly disclosing a division within the intelligence agencies may reflect his frustration that they have yet to produce a consensus on the virus’s origins, according to some former intelligence officials.

Biden initially asked the intelligence community to examine the origins of the pandemic in March. About two weeks ago, he received the results of that inquiry in his presidential daily briefing, according to a White House official, who spoke on the condition of anonymity to discuss sensitive material.

As Biden and his top aides digested the results of the initial intelligence review, senior White House officials realized there were more questions they could try to answer. And two former senior officials told The Washington Post that they believed the intelligence community under Trump had not sufficiently examined all of the material that may shed light on the origins of the pandemic.

Biden then requested a declassification of at least some of the initial results, which showed the intelligence community was split on whether the virus came from nature or the Wuhan lab.

The president said Wednesday that two “elements” of the intelligence community “lean toward” the hypothesis that the outbreak began when an animal infected a human, while another leans toward the notion of a lab escape, “each with low or moderate confidence.”

The president decided to take the unusual step of revealing an inconclusive debate among the intelligence agencies because of the public interest in understanding the origins of the virus, but also because of the “swirl around the issue,” said the White House official, referring to the renewed public conversation around the lab theory.

Another factor, the official said, was China’s decision to signal at a World Health Assembly meeting that it would not support the next steps in an international investigation into the origins.

“China’s part has been completed,” a delegate from China said at the meeting.

That dismissal “accelerated and intensified our desire to declassify what we knew from our own investigation, and share it as quickly as possible,” the White House official said.

Experts who have pushed for more scrutiny of the lab-leak hypothesis applauded Biden’s call for more investigation. Jamie Metzl, a National Security Council staffer in the Clinton administration and a member of a World Health Organization advisory panel, called the statement “solid and reasonable.”

Still, Metzl, who helped organize an open letter calling for more scrutiny of the Wuhan lab unrestricted by Chinese authorities, said the statement “does not go far enough” in calling on the World Health Assembly to mandate a full investigation before its annual gathering ends Monday.

Biden’s statement came after congressional Republicans had been pressing the lab theory with renewed vigor for several months.

Sen. Rand Paul (R-Ky.) pressed Anthony S. Fauci, the government’s top infectious-disease expert, when he appeared before a Senate panel earlier this month, saying that government officials have been unequivocal about insisting the virus was not man-made.

“I’m fully in favor of any further investigation of what went on in China,” Fauci responded.

Sen. John Neely Kennedy (R-La.) pressed Fauci at a different hearing, quizzing him about why the United States provided funding via a subcontract to the Wuhan lab. Fauci responded that China is a logical place to get funding to study coronaviruses, since that is where they have emerged.

About two months ago, Republican leaders of the House Energy and Commerce Committee began seeking details about the pandemic’s origin from federal agencies and U.S.-funded scientists who have collaborated with Chinese researchers.

They wrote an 11-page letter posing more than 30 questions and requesting a range of documents from the National Institutes of Health, largely regarding work done by recipients of NIH funding that have collaborated with the Wuhan lab.

The lab has experimented with many species of bats and the multiple strains of coronaviruses they carry, and the first covid-19 cases were reported in late 2019 in and around the city of Wuhan.

Although aides to the House Republicans sought over a period of weeks to enlist bipartisan support for an inquiry into the cause of the pandemic, none of the Democrats stepped forward. But on May 14, one Democrat, Rep. Anna G. Eshoo (Calif.), who is chairwoman of the Energy and Commerce panel’s health subcommittee, broke ranks.

Eshoo issued a statement applauding a group of 18 scientists who had just written, in the journal Science, that a “transparent, objective, data-driven’’ investigation is needed to determine the pandemic’s origin. “If you take partisan politics and mix them with science, it’s a toxic combination,” Eshoo said in an interview. “One doesn’t go with the other.”

Yasmeen Abutaleb, Adam Taylor, Katerina Ang and Erin Cunningham contributed to this report.