White House advisers, in an apparent election-year gambit, barred Interior Secretary Cecil D. Andrus from warning Congress yesterday of a possible veto of a controversial reclamation bill sought by powerful farm interests in the West.

Andrus had included his veto warning in an 11th-hour letter attacking the bill now before the House Interior Committee. The secretary, out of town yesterday, was reported by sources to be furious over the weakening of his text.

In a hearing room packed with lobbyists for the western interests, the committee yesterday began final drafting of the measure that assures more federal water subsidies for big farmers in California and other key western states.

The oncoming election has made the water issue even more sensitive than usual with the westerners, who have railed at President Carter's water policies since he took office in 1977.

The mini-flap over Andrus' letter marked the third time in less than a week that the western water issue had been raised pointedly at the highest political levels.

Andrus, himself a westerner, has been the administration's point man in its attempts to reshape water policy and curb the huge federal water subsidies that many irrigators receive in 17 western reclamation states.

But the secretary's spear was blunted a bit Monday when White House domestic advisers Stuart E. Eizenstat and Bert Carp toned down his letter to Congress.

The version they approved left no doubt of administration opposition to the pending bill.

But excised from the four-page letter was a sentence in which Andrus said that if Congress ultimately passed a bill similar to the pending version, "I will have no choice but to recommend that such a bill be vetoed."

Ironically, Andrus previously had issued a similar warning publicly 10 days ago in a speech in Berkeley, Calif., apparently without arousing the White House.

Carp refused to comment on "internal workings" of the White House, but he said yesterday that "it was the judgment of people who reviewed the letter that it wasn't going to be constructive to threaten a veto."

"We have a general policy of not threatening vetoes," Carp said. "Whoever leaked this doesn't do a service to the cause of justice . . . We would like to focus public attention on the manifold deficiencies of that bill."

While Carp denied that campaign considerations entered into the decision to tone down Andrus, the administration has been receiving clear signals of political concern from outside allies.

At a luncheon last Wednesday to organize farm forces for the Carter-Mondale ticket, guests touched on their western water concerns with administration and campaign officials.

Several persons who attended the sesson said they heard Evan S. Dobelle, Carter-Mondale finance chairman, say a California congressman had offered to hold a fund-raising breakfast if the president would approve the reclamation legislation.

"No one heard me say that," Dobelle said yesterday. "I don't deal in that. I don't deal in substantive issues. I was there, I spoke and I said Rep. Tony Coelho had brought the water issue to my attention."

Dobelle went on to say, however, that Coelho (D-Calif.) had offered to stage a July 4 breakfast in Fresno for the Democratic National Committee -- not the Carter-Mondale organization -- if the president could attend.

Although not a member of the Interior Committee, Coelho is the leading advocate in Congress for the big growers in the San Joaquin Valley whose crops are irrigated with the federal water.

Most of the abuses and amassing of large land holdings in violation of the 1902 reclamation law that Congress is now revising have occurred in the valley.

Coelho's version was that he did not offer or agree to put on a fund-raiser.

He said he had been approached by the DNC, but had not yet decided if he would do so. Coelho said, "It's none of your business why I have not decided." He also refused to explain why he had discussed water issues with Dobelle.

Dobelle denied any connection between Coelho's fund-raising efforts and the legislation. "There was no quid pro quo. We simply don't do that," he said. "But wherever, you go, and there is a local issue, you hear about it. We have had Bob Strauss and Mrs. Rosalynn Carter in the Fresno area. We heard about water. Strauss heard about water when he was out there."

Several DNC-orchestrated fund-raising events last year in the Fresno area represented by Coelho drew strong support from Republicans who frankly conceded they hoped to influence the Carter administration with their money.

As Dobelle and others were talking to farm interests at the Carter-Mondale luncheon, the president himself was being lobbied to oppose the pending reclamation bill.

George Stone, president of the National Farmers Union, which represents about 300,000 farm families, met with Carter at the White House to talk about agricultural policy.

"I told the president I had been out to Berkeley and I heard Secretary Andrus warn of a possible veto," Stone recalled yesterday. The president said that Andrus had communicated to him about the reclamation bill. Then I went into the giveaway aspects of the bill and Mr. Carter certainly did not say he disagreed with that. I felt he was more favorable toward a veto."

Stone said the farmers union opposes the House bill because "it legalizes all the abuses and illegal things that have been done in the past in the name of the Reclamation Act of 1902. We hope it gets buried and doesn't even get to the White House."

In his letter to the House committee, Andrus touched on similar points, saying the pending bill would be a "setback to family farming" by encouraging big farmers to corner the use of federally subsidized water "at bargain rates."

Andrus noted that the 960-acre limit that he and the administration support, and which is in the bill, would allow 97 percent of present reclamation farm operations to continue unchanged.

But, he said, 3 percent of the operators control 30 percent of the more than 11 million acres of irrigated land, and the legislation, if not changed, would strengthen their grip on the water subsidies.

The 1902 law the bill would revise sets a limit of 160 acres of federally irrigated land that a farmer may work. Nonenforcement has allowed corporations and wealthy farmers to amass holdings that run into the thousands of acres.