In a matter of a few minutes on the floor of the House of Representatives, the landscape of the fall campaign was changed.

It happened last Thursday when House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) lumbered to the well and had the incomparable experience of lecturing the opposition on the subject of loyalty to Ronald Reagan.

Seldom does statesmanship come in the form of sweet revenge. In the dead still of the chamber, O'Neill instructed the younger House Republicans on their debt and duty to their leader.

"You are here because of Reagan. You are here because he was elected president of the United States. He brought you to the Congress of the United States, and now we are on the eve of an election and he is asking for a change of policy. Are you going to follow the leader that brought you here, or are you going to run? I ask you just to think of that."

If he could put aside his differences with the president ("We seldom agree," O'Neill said), they could, he told them. One hundred and three Republicans gritted their teeth, and voted for the bill Ronald Reagan had to swallow so hard to accept.

For O'Neill, as for the president, it was a case of assuming one burden and laying down another. For himself and his party, O'Neill took the onus of providing critical support for a tax increase. But he was also bowing out as a campaign issue. No longer can the president use O'Neill as the epitome of Democratic messiness and extravagance.

"Now," crowed one Democrat after the vote, "the issue in the campaign is not the Democratic leadership; it's the failure of Ronald Reagan's economics."

The president prefers, whenever possible, to personalize philosophical differences. He found O'Neill -- a contrast to Reagan's trim, telegenic self -- an ideal target. Reagan tried to gloss over the chasm that opened between him and the right by attributing the rebellion led by conservative Rep. Jack Kemp (R-N.Y.) to Kemp's lust for the presidency.

For Reagan, passage of the tax bill was a triumph with two faces.

As a member of the Republican National Committee said, "It showed Ronald Reagan's strong leadership that he could pull out the vote. On the other hand, it has been attributed to a change in the direction in which he is leading the country."

The impassioned fringe on the right was sufficiently angry with him before he learned to call the tax increase a "tax reform." They are seething over his failures to push for amendments on abortion and school prayer.

But he gains in the center by having shaken hands with reality. Those who regard Reagan as an obdurate ideologue must applaud his flexibility in confronting mountainous deficits. He is not, after all, an extremist.

As luck would have it, his first outing after the indispensable and uncongenial victory on taxes has taken him to California and to the heart of the split in his party.

His old friend, Pete Wilson, the mayor of San Diego, who is in a tightening race for the Senate against Gov. Edmund G. (Jerry) Brown Jr., came out against the tax bill. Brown, who has been lagging, instantly declared himself in favor of it.

Naturally, the press made much of the paradox. Michael K. Deaver, Reagan's deputy chief of staff, told Wilson's campaign manager that the president was "disappointed" that Wilson was opposing him on such a critical issue.

"But," Deaver said, "we would still be much more comfortable with Pete Wilson than Jerry Brown in the Senate."

The Wilson campaign staff suggested that in his remarks to a $1,000-a-plate fund-raiser for Wilson last night, the president might wish to emphasize Wilson's excellent character and their long and congenial association. He hardly did so.

Wilson took the position that in his 15-year public career he has always been on the side of cutting taxes and that to change merely for Reagan's sake would create a credibility problem.

He has created a few other problems for himself, such as, for instance, thinking out loud about the wisdom of perhaps making Social Security voluntary for voters under 45.

Brown is a preternaturally agile campaigner and, through careful exploitation of Wilson's opposition to the nuclear freeze, the trendy referendum issue in California, has managed to draw even with Wilson in the southern part of the state, where Brown has always been weakest.

"This is the right thing to do," Brown declared in announcing his support of the president's tax bill.

The Democratic governor was echoing Tip O'Neill and House Majority Whip Thomas S. Foley (D-Wash.) and making the intoxicating discovery that bipartisanship can be fun, when it means underlining the fact that Reaganomics doesn't work.

What the Democrats fear most is that the voters, having seen that 40 years of Democratic spending led to near bankrupcty and that Reagan's theories have brought us to near depression, may decide there is no solution -- and decide to stay home on Election Day.