Ordinarily, it might be one of an administration's finest political moments: the chance to leave its mark on a major piece of legislation that affects virtually every American.

But it is turning out almost precisely the opposite with the Reagan administration's handling of a new farm bill, which has been debated at fever pitch all year on Capitol Hill.

Congress is moving toward passage of farm legislation with only a desultory nod toward the White House, notwithstanding repeated threats of a veto if the legislators go "too far."

"The administration became almost irrelevant during our markup," said Rep. E. Thomas Coleman (R-Mo.), a key GOP member of the House Agriculture Committee. "I have felt from the start that the White House misunderstood the farm-credit situation and the severity of things going on out there."

Reflecting this view, House Republicans defected in droves and helped defeat the administration overwhelmingly on dairy- and sugar-support amendments last week. And a few GOP members' independent thinking helped the Senate Agriculture Commitee turn out a bill that was far above White House budget demands.

But, increasingly, the administration is viewed on a bipartisan basis as being so far out of touch with the reality of problems in the farm sector that its views on economizing and market orientation get a rhetorical back of the hand.

Its lobbying efforts are viewed on the Hill as uncoordinated and unhelpful. In the most extreme example, Agriculture Department lobbyists recently dropped by the office of Rep. Stan Lundine (D-N.Y.) to ask for help on an amendment to scale down peanut supports. Lundine not only authored the amendment, his similar amendment passed the House by 91 votes in 1981.

Even among friends, Agriculture Secretary John R. Block is seen and heard as a leader giving mixed signals, whose usefulness is being questioned openly by true-blue GOP legislators. Continuing rumors of his imminent departure from the administration also may hurt him.

In the latest volley from Capitol Hill, Rep. Edward R. Madigan (R-Ill.), the ranking GOP member of the House Agriculture Committee, joined the parade of critics who hold Block responsible for some of the legislative problems.

"The president's point man in all this is Secretary Block. To be effective, he has to be perceived as an advocate of the farmers' interest. That doesn't exist," Madigan said. "And he is not being effective as the administration point man. As secretary of agriculture, the wrong perception exists and his effectiveness suffers.

"Their agenda is too large, and they are spread too thinly," Madigan concluded.

A commodity lobbyist with close ties to the White House lamented: "They don't know how to win . . . . Norton and Thompson are not even players in this one . . . and people on the Hill aren't even asking Block for help when they need it."

Norton is Deputy Secretary John R. Norton, second in command at USDA, who has played virtually no role in farm-bill deliberations. When farm bill action on the Hill was warming up in September, he was following the progress of immigration legislation that would affect him and other big western growers who rely on imported field hands to harvest their fruit and produce.

Thompson is Robert Thompson, the assistant secretary for economics, who argued the administration's point of view before the Senate committee. Members increasingly criticized him and his office for "cooking" economic data to suit the administration's philosophical beliefs.

Congressional Republicans' unhappiness with the administration's response to their pleas for Farm Belt assistance is abetted by other slights, or perceived slights. Rep. Ron Marlenee (R-Mont.), another ranking Agriculture Committee member, complained to party colleagues that the administration turned to a liberal Democrat, Rep. Thomas Downey (D-N.Y.), when it wanted help in cutting back the sugar program.

"There were people on the committee with far better records of support for the president than Tom Downey," a Marlenee aide said.

How far and how much all of this rubs off on Block is the imponderable.

"There are so many backs to the wall, people trying to survive," Coleman said, "that they don't spend a lot of time trying to figure if President Reagan or Jack Block is to blame."