Over four days and nearly 20 hours of testimony, under harsh questioning from Democrats, Health Secretary Robert F. Kennedy Jr. has repeatedly backed away from his longstanding criticism of the measles, mumps and rubella vaccine. On Wednesday, he made his strongest statement yet — albeit on behalf of his department and not himself.

“We promote the M.M.R.,” Mr. Kennedy told the Senate Finance Committee on Wednesday morning, referring to the combined vaccine for measles, mumps and rubella. “We have advised every child to get the M.M.R. That’s what we do.”

The comment stands in stark contrast to Mr. Kennedy’s past advice, and senators wondered aloud why he hasn’t told the public what he said on Capitol Hill this week. Last week, he conceded the measles vaccine is “safe and effective” for most people.

When measles broke out in Texas last year, Mr. Kennedy did not recommend vaccination; he said it should be “a personal choice.” Last year, asked if he would advise parents to vaccinate newborns, he said it was not up to him to provide medical advice. His advice, he said, was: “Do your own research.”

But even as he shifted on measles, Mr. Kennedy stuck by his longstanding assertion that improvements in hygiene and sanitation, and not vaccination, fueled the decline in deaths from infectious diseases during the 20th century.

“If you want to talk about what, why disease mortality has disappeared in the 20th century, it was not vaccines,” he said, testifying before the Senate health committee Wednesday afternoon.

As proof, Mr. Kennedy cited a study published in the journal Pediatrics in 2000. But he failed to note that the study also reported that vaccines introduced in the second half of the 20th century had “virtually eliminated” deaths from diseases including polio and measles. In 1999, the Centers for Disease Control and Prevention listed vaccination as one of “ten great public health achievements” of the 20th century.

After Mr. Kennedy made the assertion, Senator Bill Cassidy, the Republican chairman of the Senate health committee, asked about the author of the study; Mr. Kennedy gave him the author’s name. Later in the hearing, Mr. Cassidy produced the paper and told Mr. Kennedy he had taken it out of context.

House Republicans had big plans for Earth Day this year: They would pass a bill to narrow protections for endangered species that they had long seen as federal overreach.

It didn’t work out that way.

On Wednesday afternoon, Republican leaders suddenly canceled a vote on the measure after an initial procedural vote showed shaky support from party members. One Florida Republican, Representative Anna Paulina Luna, publicly aired concerns about the bill before the scheduled vote, writing on social media: “Don’t tread on my turtles. Protected means protected.” Her post contained an image of a yellow flag emblazoned with a sea turtle and the slogan “Don’t tread on me,” a phrase dating to the American Revolution that some conservatives have embraced in recent years.

The flip-flop on Wednesday was an embarrassing setback for Speaker Mike Johnson of Louisiana. And it left uncertain the fate of the ESA Amendments Act, a sweeping bill that would limit protections for species whose populations are beginning to recover, among a slew of other changes.

The bill’s lead sponsor, Representative Bruce Westerman, Republican of Arkansas and chairman of the House Committee on Natural Resources, said he was trying to shore up support in the hopes of rescheduling a vote on the measure. “We just have a few provisions we’ve got to work through on it, and hopefully in the next couple of weeks, we’ll be able to vote on it,” he said.

Representative Kat Cammack, a Florida Republican, said she had raised concerns about a provision in the bill that would allow state and federal officials to exempt certain activities from Endangered Species Act restrictions. She said she worried that officials would codify an exemption that the Trump administration recently granted for oil and gas drilling in the Gulf of Mexico.

“I have coastline in my district,” Ms. Cammack said, citing the possibility of an oil spill sullying her state’s beaches. “I want to make sure that we’re doing everything that we can to be the best stewards as possible.”

Before the vote was postponed, conservationists had warned that the bill could speed extinctions and risk the recovery of numerous species, including piping plovers, black-footed ferrets and North Atlantic right whales. And they called the planned timing of the vote, on Earth Day, a cruel stunt.

“It’s a slap in the face to the American people and all the wildlife they love, and the ecosystems that support our lives,” Mary Beth Beetham, director of legislative affairs at Defenders of Wildlife, an advocacy group, said on Wednesday morning.

A few hours later, she was rejoicing.

“Now we can really celebrate Earth Day!” she said in a statement after the measure was pulled from the House floor. “The public defeat of the Westerman bill is a direct result of sustained constituent pressure. Congress is finally listening to the majority of Americans who support the Endangered Species Act, rather than centering politics and money in its policy decisions.”

Republican supporters countered that the Endangered Species Act needs a serious overhaul. They said the bill would make it easier to remove unnecessary protections from gray wolves, grizzly bears and other predators whose populations have rebounded in certain areas over the past several decades.

“Folks in my district have an incredible frustration regarding the gray wolf population because they have recovered,” said Representative Michelle Fischbach, Republican of Minnesota, during a hearing on the bill on Monday. She said that gray wolves had killed cattle as well as “family dogs tied up in the front yard.”

The planned vote was the latest recent effort by congressional Republicans to erode environmental protections.

Last week, the Senate voted to allow mining upstream from Minnesota’s Boundary Waters Canoe Area Wilderness, one of the country’s largest and most visited expanses of federally protected lakes and forests, sending that measure to President Trump to be signed into law. And the House approved three bills that would narrow the reach of the Clean Air Act, although their fate in the Senate remains uncertain.

At the center of the debate over the Endangered Species Act are two polarized views of the law. Democrats and conservation groups tend to celebrate it for preventing extinctions, noting that less than 1 percent of species protected under the act have been lost. But many Republicans criticize the law for recovering only a small number of species to the point of removing them from the list.

The bill that the House had aimed to pass Wednesday would make a number of changes to the law. It would require regulators to conduct economic and national security analyses when determining whether to list a species as endangered, while limiting their ability to consider future impacts, such as climate change. It would also weaken requirements that the federal government limit harm to endangered species, reduce certain habitat protections and cap fees awarded to lawyers in endangered species litigation.

The first Earth Day, in 1970, came in response to a series of environmental disasters. The pesticide DDT was devastating bird populations. A record-breaking oil spill had polluted the waters off Santa Barbara, Calif. In Cleveland, the Cuyahoga River had caught on fire.

For the initial commemoration, Congress effectively closed down so that lawmakers could attend events. More than 20 million Americans participated in rallies, lectures and protests across the country, including at more than 1,500 college campuses and 10,000 schools.

The public outcry galvanized the modern environmental movement. It also spurred Congress to create the Environmental Protection Agency and to enact three landmark environmental laws within three years: the Clean Air Act, the Clean Water Act and, finally, the Endangered Species Act.

Megan Mineiro contributed reporting from Washington.