Office of Management and Budget Director David A. Stockman was poised at the witness table waiting to tell the Senate Small Business Committee why it should abolish the Small Business Administration when there was a pause for a commercial from the committee -- a film of President Reagan praising the agency only two years ago.

"We at the White House have come to enjoy old films of the president," said Stockman in attempting to shrug off the not-so-subtle message from Republicans and Democrats that any obituaries for SBA are premature. The laughter died quickly, however, and the message became more pointed.

"You'll lose, you'll lose, not because I've got the votes but because I've got the case," claimed committee Chairman Lowell P. Weicker Jr. (R-Conn.).

The scene is repeated daily on Capitol Hill as committees with jurisdiction over programs targeted by Reagan for cuts or annihilation rise up to defend them, building a wall of resistance to efforts to put together a massive deficit reduction package for the next three years.

Backed up against this wall, Senate Republican leaders have stalled in their effort -- proclaimed with heady optimism from the steps of Blair House only two months ago -- to write a deficit-reduction package on their own.

And the Senate Budget Committee is expected to face the same wall as it takes over the chore next week, attempting to write a budget resolution encompassing the deficit reductions that the leadership failed to agree upon.

Contending he could wait no longer for a consensus on spending cuts, Budget Committee Chairman Pete V. Domenici (R-N.M.) said yesterday the committee would go ahead with its budget resolution markup as scheduled on Monday, with hopes of finishing by Wednesday or Thursday.

There will be no leadership plan from which to work. Their only guidance will be a blueprint based on a spending freeze augmented by a long list of specific spending cuts -- many of them proposed by Reagan -- that have raised hackles both in Congress and in powerful constituency groups across the country.

"We're a long way from having a Senate Republican position," said Sen. Robert W. Kasten Jr. (R-Wis.), a member of the budget panel. "We just have to get on with it," said Domenici.

At the Small Business Committee hearing, Stockman characterized SBA's lending operations as a "$3 to $4 billion annual program which indiscriminately sprays a faint mist of subsidized credit into the weakest and most prosaic nooks and crannies of the nation's $4 trillion economy." It serves "almost no rigorously defined public policy purpose," he said.

Defaults on loans could cost $300 million to $400 million a year, which the country cannot afford, Stockman said. Weicker challenged many of Stockman's specific claims, accusing him of making "wrong, incomplete and misleading statements" in his justifications for abolishing the program.

Sen. Dale Bumpers (D-Ark.) also challenged Stockman's contention that the program had minimal national impact, noting 10 firms that got their start with SBA loans, including Apple Computers and Federal Express, which last year paid a combined total of $200 million in federal income taxes.