For almost a year, NASA Administrator Jim Bridenstine has been scrambling to meet a White House mandate to return astronauts to the moon by 2024. He spent the summer wooing members of Congress for funding and brought in a marketing executive to help sell the public on the effort, which the agency has branded “Artemis,” after the twin sister of Apollo.

But now he is facing the toughest challenge yet. On Wednesday, the House space subcommittee is scheduled to begin consideration of a bill that flatly rejects the 2024 timeline and would, if enacted, order the agency to place priority on traveling to Mars over the moon. The bill calls for the lunar deadline to be pushed back to 2028, NASA’s original plan before Vice President Pence last year called for the agency to speed things up.

While NASA had been planning on building an outpost on and around the moon, the House bill would direct the agency to focus instead on developing the technologies to put astronauts in orbit around Mars by 2033. Instead of serving as a staging point for the moon, as NASA intends, the lunar space station, known as Gateway, would exist to test technologies in deep space needed for Mars under the House bill.

It also would favor traditional contractors, such as Boeing, which is building NASA a massive rocket, known as the Space Launch System, that it says could be used to send astronauts directly to the surface of the moon — bypassing the outpost that NASA wants to build in lunar orbit. Instead of having the private sector build a lunar lander that NASA could then use to send astronauts to the moon, the bill would require NASA to “have full ownership” of the spacecraft.

It also would dictate that the lander be integrated with an upper stage being built by Boeing for its SLS rocket. But NASA has said for months that it does not think the upper stage would be ready in time and has asked that funding for the upper stage be deferred.

The proposal has split the space community and elicited a pointed, but measured, response from Bridenstine, a Republican former member of Congress who finds himself in the precarious position of having to balance the will of Congress against the Trump administration’s 2024 mandate.

In a blog post Monday, Bridenstine wrote he was “concerned that the bill imposes some significant constraints on our approach to lunar exploration.”

In particular, he wrote that “we are concerned that the bill’s approach to developing a human lander system as fully government-owned and directed would be ineffective. The approach established by the bill would inhibit our ability to develop a flexible architecture that takes advantage of the full array of national capabilities — government and private sector — to accomplish national goals.”

The Planetary Society, the space nonprofit group founded by Carl Sagan and led by chief executive Bill Nye, said the bill “would disrupt and delay a planned return of U.S. astronauts to deep space.” It urged Congress to get out of the way and give NASA “the flexibility to best implement its efforts at the moon and beyond.” And it recommended that the committee “remove the provisions restricting activities and limiting competition for exploration capabilities.”

The bill has not yet emerged from committee and will probably be modified, perhaps significantly, before it does. “The chances of this passing in its current form are virtually zero,” said John Logsdon, a space historian who serves as a professor emeritus at George Washington University's Space Policy Institute.

But it demonstrates how precarious even NASA’s most-trumpeted plans can be and why the agency has been unable to return astronauts to the moon since the end of the Apollo program in 1972. Several presidents have proposed grand missions to the moon and Mars in the years since, only to see those efforts come up short, whether because of a lack of congressional buy-in or a change in administrations. When he was president, George W. Bush proposed going to the moon. Under Obama, NASA changed course after he said, “we’ve been there before,” and instead directed NASA to go to an asteroid and then to Mars.

Under Trump, the White House gave NASA another case of whiplash when it directed the agency to focus again on the moon.

“One can reasonably question, after 40 years of trying without success, whether the government really wants to sponsor human exploration,” Logsdon said. “Why we haven’t been back to the moon is because those in the country with the power to provide the resources to do that have not agreed on a way to go forward.”

The Trump administration put the power of the White House behind the effort in a way not seen in years. Pence, the head of the reconstituted National Space Council, has given several high-profile speeches on space. To fund the Artemis moon program, NASA is expected to soon lay out a five-year spending plan to cover a program that is estimated to cost about $30 billion. To meet that goal, next year’s NASA budget is expected to increase significantly over the current $22 billion.

House members have said they are skeptical of the plan and that they need to see a detailed timeline and funding plan before they would vote to put money behind it. In an interview Tuesday , Rep. Kendra Horn (D-Okla.), the chair of the House space subcommittee, said NASA has yet to lay out its plan in any detail or how it would pay for it.

A program as ambitious as getting to the moon or Mars “requires a clear and unambiguous plan, and we’re still waiting for it,” she said. Mars has been the ultimate destination for years, she said, and the bill “sets a clear goal for NASA to get us there in the quickest and most efficient way possible.”

She added that many different contractors would be involved in the effort, and the program would not favor any. But, she said, NASA should own the technology behind any lunar lander it uses to get astronauts to the surface “because this would be a national program and it’s in the nation’s interests to ensure we own the technology. NASA should have strong insight and oversight from day one.”

In a statement to The Washington Post last year, Rep. José E. Serrano (D-N.Y.), the chair of the House appropriations subcommittee that funds NASA, said, “The President has decided to play politics with the Artemis program by seeking to speed up plans to send humans back to the moon by 2024 instead of 2028 without a strong justification for doing so.”

Still, some think NASA and Congress will ultimately be able to find common ground. The Senate’s version of the bill is more in line with NASA’s plans, and the White House is making it a priority.

“There’s more momentum behind Artemis than there had been in the past 40 years with all the prior initiatives,” Logsdon said. “And I think if there isn’t disruption of the current plan, we’re liable to actually stumble our way through an eventual return.”

“I would like to think that there’s enough bipartisan support around both sides of Congress to get a bill taken care of,” said Dan Dumbacher, the executive director of the American Institute of Aeronautics and Astronautics. “But it’s going to be a struggle.”

The legislation is a steppingstone, he said, that has at least some bipartisan support. It was sponsored by two Democrats, Horn and Rep. Eddie Bernice Johnson (Tex.), the chair of the House Science, Space and Technology Committee, along with Republicans Brian Babin (Tex.) and Frank D. Lucas (Okla.).