Whatever its long-term consequences, the victory President Reagan won last week on the tax bill has redefined his role in the political system. The man who built a successful and long-lived part for himself as the scourge of "the Washington buddy system" (to quote the phrase he used in his 1976 presidential bid) was rescued by the congressional establishment of both parties.

His win in the House was accomplished with more Democratic than Republican votes (123 to 103). The Senate vote was on more normal partisan lines. But even there, the 11 Republican defections would have defeated the bill, had not nine Democrats crossed the line to support the president.

Even more striking was the pattern of the vote. Most of the Republican defectors were from the conservative wing, where Reagan has had his spiritual home, and especially from young conservatives, to whom he has been a hero.

Meantime, it was the senior people in both parties who rallied to the president's side. In the House, Speaker Tip O'Neill spoke as powerfully for the bill as did Minority Leader Bob Michel, while Democratic Whip Tom Foley shared vote counts with Republican Whip Trent Lott. In the Senate, it was the unlikely partnerships of Alan Cranston and Howard Baker, Ted Kennedy and Bob Dole, that turned the tide for Reagan.

What did the Democrats get for their good deed? Well, they avoided the denunciations they would have received and deserved if they had helped kill the tax bill for narrow partisan reasons. As Rep. Leon E. Panetta (D-Calif.) put it, the tax bill "has the most significant reforms" in more than a decade.

But realistic Democrats do not expect any more than that. In district after district, Republicans will still run ads saying the choice is to go ahead on the new path of President Reagan or return to the old tax-and-spend policies of Tip O'Neill.

Realistically, Democrats know, too, that they have given added leverage to Reagan on the spending issues that lie ahead. The budget and tax bills passed last week contain $30 billion in spending cuts, and that is just a down payment on larger cuts to come.

Most people think the pattern of last week's vote is not likely to be repeated soon. White House Counselor Edwin Meese III said, "It is a one-time coming together, rather than a long-term coalition." Rep. Newt Gingrich (R-Ga.), one of the young conservatives who broke with Reagan, said: "I think we'll be back in two weeks, ready to fight the Democrats. If the president decides on a veto strategy this fall (to enforce spending cuts), it will be impossible for him to be allied with half the people who voted for this tax bill."

That talk is persuasive, and yet one wonders if this past week was not a psychological landmark in the Reagan presidency. For the first time, he governed against his fellow conservatives. He discovered that he could win a vital real-world victory with the help of those who recognize the realities, even when his old conservative soul mates were quoting his own rhetoric against him. It will be surprising if that disconcerting but liberating discovery does not affect his actions in the remainder of his presidency.