Mr. Rockefeller and Ms. Hutchison asked Paul K. Martin, the NASA inspector general, to “examine whether this or other recent actions by NASA were intended or could reasonably have been expected to foreclose the ability of Congress to consider meaningful alternatives” to the president’s proposed policy, which invests heavily in new space technologies and turns the launching of astronauts over to private companies.

Image Senator John D. Rockefeller IV in the Senate subway after a vote on Capitol Hill. Senator Max Baucus sat in the foreground. Credit... Luke Sharrett/The New York Times

Congress has not yet agreed to the changes and inserted into this year’s budget legislation a clause that prohibits NASA from canceling the program, called Constellation, or starting alternatives without Congressional approval.

Mr. Hanley had been publicly supported by Maj. Gen. Charles F. Bolden Jr., the NASA administrator, and other NASA officials, but he may have incurred displeasure by publicly talking about how Constellation could be made to fit into the slimmed-down budgets that President Obama has proposed for NASA’s human spaceflight endeavors.

The senators’ letter cites an e-mail message from Douglas R. Cooke, NASA’s associate administrator for exploration systems, to Mr. Hanley on May 21 that told him to focus on items that could be used in the president’s proposed space policy and put less priority on other work.

“It’s enough for us to be extraordinarily concerned,” said a Congressional staff member, who was authorized to speak only anonymously. “It’s not the smoking gun, but it’s smoking. We just want the inspector general to follow the path and report back to us what he’s finding.”