House Speaker Thomas P. O'Neill Jr. (D-Mass.), saying President Reagan has done the "greatest selling job I've ever seen," conceded yesterday that the Democrats haven't nailed down enough votes to pass their alternative to the Reagan austerity budget.

O'Neill's assessment came as Reagan continued to pick up Democratic support for his tax-and-spending plan and the House prepared to begin voting today on spending tragets for fiscal 1982, including $36.6 billion in program cuts that would be required by the plan Reagan is pushing.

"Am I lobbying?" O'Neill asked rhetorically. "The answer is yes. Am I getting commitments? The answer is no."

O'Neill had taken a similarly pessimistic view the previous Monday, only to bounce back with a good soldier's optimism after some Democrats complained he was misreading his troops. But by yesterday the Monday blues had returned. "Do I have disappointments?" he asked. "The answer is yes."

O'Neill said he has a "solid count" of 175 votes for the Democratic budget alternative, 41 votes short of what's needed.

Conservative Democrats also claimed they were giving Reagan the votes he needs to carry his budget in the Democratic-dominated House, estimating 40 to 50 probable crossovers. If Republicans vote solidly with Reagan, it would take only 26 Democratic votes to give the president a victory.

An Associated Press survey published over the weekend indicated at least 29 Democrats would support Reagan's budget, with only 1 certain Republican defection from it. After Reagan met with about 20 lawmakers yesterday, the list of likely Democratic recruits had grown to at least 33.

Among the largely but not exclusively Southern group of White House-targeted swing votes was Rep. Donald J. Albosta (D-Mich.), who told the AP he was persuaded to vote for Reagan's budget after the president assured him he would look into Albosta's complaint that Michigan suffers unfairly from funding distribution formulas.

Swimming against this tide, a group of House committee chairmen took the floor to complain that the Reagan budget, as revised with blessing by Reps. Phil Gramm (D-Tex.) and Delbert L. Latta (R-Ohio), would salvage many social programs and pervert the congressional budget process.

Education and Labor Committee Chairman Carl yd. Perkins (D-Ky.), whose committee would have to accept $11.2 billion in cuts, more than twice as much as any other committee, complained that Congress "might as well kill many of these programs" if the Gramm-Latta budget is adopted.

In addition to liberal-backed substitutes offered by the Congressional Black Caucus and Rep. ydavid R. Obey (D-Wis.), the House will be choosing between the Gramm-Latta version of Reagan's budget, which contains deeper cuts and a smaller deficit than Reagan proposed, and the Democratic alternative drafted by the Budget Committee, which also has a smaller deficit, achieved largely by anticipating a smaller tax cut than Reagan recommends.

But the major difference is that the Democratic proposal saves about $7 billion worth of social programs from Reagan's budget ax and provides for programs cuts in a less drastic way. The Gramm-Latta proposal would require congressional committees to make $36.6 billion in permanent program cuts, while the Democrats would require only $15.8 billion in permanent program cuts. The rest of the savings would be made by cutbacks in annual appropriations.

The Senate has already gone down the same path as Gramm and Latta, voting last month to require $36.9 billion worth of program cuts.

Meanwhile, Vice President Bush, in a speech to newspaper publishers, accused House Democratic leaders of attempting to "thwart the mandate of the people" in fighting Reagan's budget. Responded O'Neill: "He's a robot, right in line with his party."