The assassination attempt on President Reagan last week may make it more difficult for Democrats to pass an alternative budget in Congress, House Speaker Thomas P. (Tip) O'Neill Jr. said at a journalists' breakfast yesterday. It has "held up the efforts of the Democrats. We're presenting our budget at an inopportune moment," he said.

O'Neill said that on the Thursday and Friday before the attempt, his mail turned against the Reagan budget proposals 457 to 340, whereas before that it had run "1,000 to 1 for Reagan" in what O'Neill called an "engineered campaign" -- 3,000 letters from Houston, 1,500 from Fort Lauderdale and a few thousand more from California.

The Senate has passed Reagan's budget virtually intact. The Democratic-controlled House, however, is moving more slowly. Reagan's tax package will have a harder time in both bodies, O'Neill said, adding that only three members of the Ways and Means Committee would vote for it today.

O'Neill said he was confident the Democrats had put together a budget coalition that includes all factions of the party. "We've got the agreement" of all Democratic conservatives, except Phil Gramm of Texas, he said, adding that he planned to allow a separate vote on defense expenditures to meet conservative objections to the Democratic budget.

The speaker added that support for Reagan is wavering among conservatives who have heard protests from their constituents that school lunch programs and other services could disintegrate as a result of the cuts. The Democratic alternative would get rid of "waste and fraud," he said, but "where the president wants 25 percent cut, we're giving him 10 percent. You can't go in with a meat-ax."

O'Neill said he knew he had been criticized by many Democrats for refusing to meet Reagan head on. But as a matter of strategy, he said, "you never attack a man when he's as popular as this president of the United States." Nonetheless, since Reagan's popularity began to dip two weeks ago, O'Neill had decided "to take off the gloves" in a speech to the Building Trades Council, he said. When Reagan was shot after a speech to the same group, O'Neill changed his mind.

Despite his philosophical disagreements with Reagan, O'Neill said that "he's tremendously disarming. I just like him. Every time I go over [to the White House] he has an Irish story about when he campaigned for Truman with Pat O'Brien or when he played the Gipper with Pat O'Brien. As an individual, I think he's terrific."

But the speaker waxed eloquent about his dedication to the programs of the Democratic Party "which built America," and which Reagan now wants to cut.

Calling himself a "progressive liberal," he confessed, "I've been one of the big spenders of all time. There's no question about that. I remember one day putting $160 million in the budget for cancer in women's breasts because Mary Lasker came down to see me with some other [experts.] We added it in committee. . . .

"I remember putting $18 million in for knock-knees in children. . . . I used to be able to sneak 'em in without anyone knowing about 'em. But it's a new ball game now. . . .

"We're the wealthiest, freest democratic nation in the world. . . . Programs like these are going to die on the vine now. But that doesn't mean that what we have done over the years wasn't right."