When Rep. Jack Brooks (D-Tex.) held a hearing three weeks ago on the decades-old idea of elevating the Veterans Administration to a Cabinet-level department, neither the VA nor the Office of Management and Budget bothered to send anyone to testify.

That was a decision that VA Administrator Thomas K. Turnage and cost-cutters in OMB may regret, several leaders of the influential veterans lobby said yesterday.

Rep. G.V. (Sonny) Montgomery (D-Miss.), chairman of the House Veterans' Affairs Committee, and Charles E. (Butch) Joeckel Jr., executive director of the Disabled American Veterans, said yesterday that the legislation, which now seems certain to clear the Senate, will not mean the automatic elevation of Turnage to head the new department. They also predicted that Cabinet status will give the VA increased power to resist more of the OMB-inspired budget cuts that have left many veterans groups unhappy with the Reagan administration.

Montgomery and Joeckel differed over what persuaded President Reagan, who had opposed additional Cabinet positions, to switch and endorse Brooks and Montgomery's legislation to transform the VA into the Department of Veterans Affairs.

Montgomery said he was told by White House staffers that Reagan, a World War II-era veteran, made the decision without consulting OMB.

Joeckel said it was pure politics. "The Democrats were pushing the idea and getting ready for the election and the Republicans don't want to be seen as opposed to it," he said.

A White House official said yesterday that the president felt strongly about the issue and believes it would demonstrate the administration's "symbolic concern" about veterans issues without increasing operational costs. There was also pressure in favor of the action from Vice President Bush and a belief that the bill had so much support on Capitol Hill that it would pass anyway, the official said.

Whatever the reason, the president's decision to support the bill, announced on the eve of Veterans Day, helped pluck the measure from obscurity and put it on a legislative fast track. The bill cleared the House Tuesday by a vote of 399 to 17 and the Senate Governmental Affairs Committee has promised to hold hearings on it Dec. 9. Proponents assume it will pass, perhaps this year.

Sen. Alan Cranston (D-Calif.), chairman of the Senate Veterans' Affairs Committee, has announced that he opposes the 11 new political appointments the bill would give the president, an aide said. But no one in the Senate yesterday was predicting that the bill will face trouble.

The House bill requires the new department to become operational within six months of the law being signed. VA officials yesterday said the measure would require relatively little action and only $30,000 a year in additional funds.

One-third of those funds would be used to boost the annual pay of the VA chief to $99,500 from the $89,500 paid Turnage, the agency's 13th administrator.

Joeckel said that Turnage, who took office March 24, 1986, has failed to earn the secretary's job "based on his performance thus far."

"He has not in our opinion done things to be an advocate for the veteran," Joeckel said. "He has pretty much toed the lines that the administration and OMB wanted. He was silent on the Cabinet. He was silent on cuts in the budget."

Montgomery said he too doubted that Turnage, a retired major general, would be the administration's choice to head the new department. "He certainly should be considered," Montgomery added.

At the time Turnage became VA chief, there was strong pressure from veterans groups to have a Vietnam veteran head the agency, which has been long accused of slighting the needs of Vietnam-era veterans. Turnage is a veteran of World War II and Korea.

"For the last couple of months, this bill has had a life of its own," said Don Smith, a VA spokesman. "I don't know what was driving it."

Montgomery said the legislation has been around the House for 35 years. He credited Rep. Gerald B.H. Solomon (R-N.Y.), ranking minority member of the Veterans' Affairs Committee, with lining up support for the bill, and Brooks, Government Operations Committee chairman, with deciding to push the measure onto the floor.