NASA Administrator Mike Griffin sympathized with Capitol Hill’s dissatisfaction with the agency’s $16.8 billion spending plan for 2007, but defended it in separate U.S. Senate hearings as the best-available course barring a cash infusion that lawmakers concede might not be in the cards.

The plan leaves a four-year gap between the retirement of the space shuttle in 2010 and the fielding of its replacement, the Crew Exploration Vehicle (CEV), while deferring several science missions and paring back the agency’s aeronautics research portfolio. It reflects a White House funding allocation that is roughly $1 billion less than anticipated in 2004 when U.S. President George W. Bush unveiled plans to return astronauts to the Moon by 2020.

“It’s obvious we need more money to fund NASA,” said Sen. Richard Shelby (R-Ala.), chairman of the Senate Appropriations commerce, justice and science subcommittee, which held its first NASA hearing of the year April 26.

Shelby’s point was seconded by Sen. Barbara Mikulski (Md.), the subcommittee’s senior Democrat, who chided the White House for not backing up its Vision for Space Exploration with a bigger budget increase for NASA. “If we are going to have a balanced and robust space program, we need more leadership from this administration,” she said.

Mikulski said she would work with Shelby “to see how we can help NASA’s top line so we can try to rebalance this budget.” But she also suggested that NASA’s plan as it now stands tilted too heavily in favor of human spaceflight at the expense of science.

When Griffin urged Mikulski and Shelby, the only two senators present at the appropriations panel hearing, not to “rob Peter to pay Paul” by taking funds from the CEV program “to pay for science missions, ” Mikulski’s reply was blunt.

“You robbed Paul to give it to Peter and you are telling us not to rob Peter to give it back to Paul,” Mikulski said. “We don’t see it as robbing. We see it as a give-back.”

When he took over as NASA administrator just over a year ago, Griffin pledged to shave three or four years off the CEV’s development time without cutting “one thin dime” from the agency’s science and aeronautics programs. But he acknowledged early this year that NASA cannot afford to accelerate the CEV given the agency’s budget outlook and the funding demands of the space shuttle and space station programs, which had been underestimated by $3.9 billion through the end of the decade.

To cover the shortfall, Griffin cut $2.3 billion from what the agency intends to spend on science through 2010 and $1.6 billion from its exploration systems budget, which includes the CEV. As a result, some science missions have been deferred or canceled, and Griffin’s plan to accelerate the CEV has been shelved. NASA now says it expects to field the CEV no later than 2014, as originally planned .

Concerns about the four-year gap in NASA human spaceflight operations dominated an April 25 hearing of the Senate Commerce science and space subcommittee , where Sens. Kay Bailey Hutchison (R-Texas) and Bill Nelson (D-Fla.), the panel’s chairwoman and ranking member, respectively, said they, too, would fight for a bigger NASA budget.

Hutchison, who also serves on the Senate Appropriations Committee, said she would continue “to urge an expanded total funding level of NASA.”

“What is concerning me is that the president’s budget request is creating a situation in which the vision for exploration and the acceleration of CEV could do away with many of the other priorities I think we share, and I don’t want that to happen,” Hutchison told Griffin during the hearing.

Nelson said NASA should be funded at the level Congress authorized — but did not appropriate — for 2007 : $17.892 billion, or $1.1 billion above the White House request.

“So if the Congress’ decision, since we are the funders, is that we appropriate the same amount NASA is authorized in the ’07 fiscal year, then it gives you new opportunities to do some things,” Nelson said.

Asked by Nelson how NASA would allocate a $1 billion-plus increase over and above the White House request, Griffin declined to answer on the spot, but said he would take the question for the record.

Whether NASA’s supporters in Congress can realistically expect to get a $1 billion increase for the agency is far from clear. Congress has yet to pass a budget resolution setting the overarching spending guidelines appropriators will have to work within to produce funding legislation this year. Even so, Mikulski, one of NASA’s most reliable and effective congressional patrons over the years, expressed doubts , pointing out that her subcommittee also is wrestling with unpopular cuts in the Department of Justice budget.

Appropriators in the Senate as well as the House would have to weigh any increase for NASA against restoring funds cut from two popular local law enforcement initiatives. The White House’s 2007 budget proposal cuts the Community Oriented Policing Services program by 78 percent and eliminates a $327 million local law enforcement block grant program.

Washington sources following the budget process said finding an additional $1 billion-plus for those two initiatives is a top priority for lawmakers up for re-election this year.