Two testy senators, one of them a Democrat, are using the Senate’s power to block presidential nominations and creating major headaches for the Biden administration and its ambitious efforts to advance liberal climate and abortion agendas.

One of them, Senator Manchin, promised on Wednesday to block all of President Biden’s nominations to posts at the Environmental Protection Agency until the administration backs off plans to use its regulatory cudgel to hobble power plants that use coal to generate electricity.

The other, Senator Tuberville, an Alabama Republican, has for months been holding up the promotions of military officers unless the Defense Department agrees to rescind plans to give servicewomen time off and reimburse their travel expenses if they need to go to another state to obtain an abortion. Mr. Tuberville has said the policy violates a law that forbids federal funding of abortion and was enacted without the approval of Congress.

“This is about not forcing the taxpayers of this country to fund abortion. That’s been a bipartisan consensus for more than 40 years,” Mr. Tuberville said on the Senate floor. “The American taxpayer is on the hook to pay for travel and time off for elective abortion.”

Mr. Tuberville’s boycott has drawn howls of protests from senior officials at the Pentagon and Democrats in the Senate, a group of whom released a letter from the secretary of defense on Wednesday calling the senator’s actions “irresponsible” and “unprecedented.”

“Delays in confirming our general and flag officers pose a clear risk to U.S. military readiness, especially at this critical time,” the defense secretary, Lloyd Austin, wrote in a letter to Senator Warren, chairwoman of the Senate Armed Services Committee’s subcommittee on personnel. “This indefinite hold harms America’s national security and hinders the Pentagon’s normal operations.”

So far, almost 200 promotions have been held up by Mr. Tuberville’s protest. Normally, senior military nominations are approved in bulk by the Armed Services Committee and proceed to the floor for votes without delay. A single senator, though, can pause the process by forcing the nominations to be considered one at a time. 

Some of Mr. Tuberville’s fellow Republicans have even stepped up with concerns about the freshman senator’s tactics. The Senate minority leader, Mitch McConnell, broke with his Alabama colleague Wednesday, telling reporters on Capitol Hill that he does not support the hold on military nominations. Last week, a bipartisan group of former defense secretaries also weighed in, saying in a letter to Senate leaders that the delay “risks turning military officers into political pawns, holding them responsible for a policy decision made by their civilian leader.”

Mr. Manchin’s protest is not likely to be as powerful as that of Mr. Tuberville, but highlights the West Virginia Democrat’s growing frustration with the Biden administration’s heavy-handed approach to enacting its climate policy without congressional approval. The senator, whose support was crucial in the passage of Mr. Biden’s highly touted climate bill last year — branded the “Inflation Reduction Act” — has since said he would vote to repeal the law if given a chance.

The latest outrage from the administration, Mr. Manchin said Wednesday, is a plan by the EPA — expected to be announced Thursday — to impose strict new emissions standards on fossil fuel-burning power plants. The measure would likely force the closure of many coal-burning power plants in Mr. Manchin’s home state.

“This Administration is determined to advance its radical climate agenda and has made it clear they are hell bent on doing everything in their power to regulate coal and gas-fueled power plants out of existence, no matter the cost to energy security and reliability,” Mr. Manchin said. “I fear that this Administration’s commitment to their extreme ideology overshadows their responsibility to ensure long-lasting energy and economic security and I will oppose all EPA nominees until they halt their government overreach.”

At the moment, there are only two nominations to EPA positions pending before the Senate, one for the head of the agency’s air office and another for the head of its enforcement division. Mr. Manchin was widely expected to oppose at least one of those nominations, that of Joe Goffman at the air office, because of the nominee’s past work writing climate-related regulations during the Obama administration.