A hamburger cookout and some low-key lobbying this weekend earned President Reagan admiring comments but no new commitments to his tax-cut package as his administration prepared for its toughest congressional battle over its economic recovery program.

Reagan returned yesterday from a day of courting 15 crucial Democatic members of Congress at Camp David, where he had hoped to persuade them to approve a bill calling for a 25 percent tax cut over three years. Many of his guests had defected to the president during the last budget vote.

But the Democrats who went up the mountain uncommitted yesterday said they came down the same way, and the president's supporters acknowledged it will be a close vote.

"We think we're going to win yes," said Treasury Secretary Donald T. Regan, who also attended the Camp David barbecue.But he added that the White House "was not counting on anything. We're still working."

The campaign climaxes at 8 p.m. tonight when Reagan will address the nation in a televised speech to whip up public support for his tax cut plan. Calls and telegrams from the president to targeted congressmen are also planned.

The White House and the House Democrats last week engaged in a kind of can-you-top-this bidding contest of adding sweetners to their tax cut proposals, including benefits for the oil industry and tax breaks for commodity dealers to whet the appetites of swing-vote southern Democrats.

It is still unclear whose plan will be sweet enough. White House officials say Reagan's forces so far are behind but are gaining momentum.

Kent R. Hance (D-Tex.), who has announced his support of the Reagan tax bill, said the Camp David session "went very well" and that the president "has definitely changed in the last three days. It's definitely going to be a close vote."

After the congression queued up for lunch, the president "reemphasized some things he's said hundreds of times" and stressed "that this is the last leg of his economic recovery program. This is vital to getting the economy moving," Hance said the president told them.

"Some of the people that went probably were maybe leaning in his direction," Hance said. "They went up feeling neutral and maybe left leaning in his direction."

Thomas J. (Jerry) Huckaby of Louisiana said he was not decided but is leaning toward the Republican proposal.

Charles Hatcher of Georgia said he "was very much impressed," but not impressed enough to commit to the president's tax plan.

"I have not made a commitment," Hatcher said. "One or two people said they would commit to him but I don't know if they already had."

Hatcher said he would have to see what other sweeteners are offered in the president's and the House Democrats' bills before deciding which he will choose. "We've been having changes fast and furious you know," Hatcher said. "I guess they're through now, though."

Charles E. Bennett of Florida said the cookout provided "more information to indicate to me I wouldn't vote for a tax cut" whether it is backed by Democrats or the president. He said he probably would not vote for any tax cut and he has told Reagan so at least three times.

Richard C. Shelby of Alabama told reporters he would vote with the president, but the Democratic leadership already has counted him as a defector. Rep. Glenn English of Oklahoma said he remained uncommitted. Other participants could not be reached for comment.

Hatcher said Reagan asked for questions about his tax cut plan. The president was asked which was the biggest tax cut -- "yours or Ways and Means?" Hatcher said, and added, laughing, "He said his was, of course."

According to Hatcher, Reagan also was asked to counter allegations that his tax cut plan favors the rich and the Democrats' program favors the poor. Reagan responded that the Democrats' plan might benefit the working class more if they planned to live only two years, a response the president made in a pitch to Congress Friday.

Other Democrats attending the outing were Eugene V. Atkinson of Pennsylvania, Bevery B. Byron of Maryland, Dave McCurdy of Oklahoma, Billy Lee Evans of Georgia, Ralph M. Hall of Texas, W. G. (Bill) Hefner of North Carolina, Romano L. Mazzoli of Kentucky, Ronald M. Mottl of Ohio and Ike Skelton of Missouri.