For more than a generation, the Republicans of New York and Connecticut have been the bedrock of the GOP's "Eastern Establishment." They appear ready to write Tuesday the political obituary of a scion of the establishment and to certify the probable nomination of Ronald Reagan, the hero of the southern and western conservatives who have long been their antagonists.

George Bush, who father, Prescott Bush, was not only senator from Connecticut but an ally of Thomas E. Dewey and the New York Republicans in the nomination battles of the last generation, is waging an uphill struggle against Reagan here in his home state. Reagan's managers are confidently predicting victory, and Bush campaign aides say there is only an outside chance that his closing five-day blitz may capture enough of the large undecided vote to reverse the apparent odds.

In New York, the county Republican organizations that were held in tight control by Dewey and Nelson A. Rockefeller when they were governor are now tumbling over each other in their eagerness to get aboard the Reagan bandwagon. The former California governor's foray through upstate New York Monday will see him collecting delelgate pledges as a conquering general receives tribute from a subjugated province.

Local GOP leaders in Syracuse and other cities on the Reagan tour are expected to follow the example of four of the five top New York City Republicans and the GOP hierarchy on Long Island by shifting their unpledged slates behind Reagan.

While the bulk of New York State's 123 Republican delegates will be elected as uncommitted Tuesday, this measure of the revolution that has taken place is that Reagan already has more assured votes from the state than Bush has delegates running there. Thirty-six of Reagan's own delegates or those of local GOP organizations that have endorsed him are unopposed on the ballot, while Bush has 34 delegates filed.

In terms of psychological effect, a Reagan victory in Connecticut Tuesday would have even greater impact, because it is here, on his family ground, that Bush is making what some in his organization acknowledge could be his last stand.

Proportional representation rules in the state's first-ever GOP presidential primary assure that the 35 delegates will split among Reagan, Bush and Rep. John B. Anderson. The rules prevent crossover votes by independents or Democrats, and Anderson's late-starting campaign is aimed at what campaign manager Sid Gardner calls "a healthy third-place finish." The Anderson effort is described by Bush partisans as "a spoiler" campaign that may deny Bush a desperately needed victory.

"If this were just Bush and Reagan, I'd have no hesitancy saying we'd win," said Malcolm Baldridge, Bush's state chairman. "In this state, a vote for Anderson is really a vote for Reagan."

There was support for that view in a poll released Saturday by the University of Connecticut, showing Reagan 24 percent, Bush 17 and Anderson 12 percent, with a huge 47 percent of the Republicans still undecided. The belief in all three camps is that most of the still-undecided voters will skip the primary.

Realizing how much he has at stake, Bush has worked furiously the last five days, touring every area of the state in a motorized van, making 35 appearances and lashing Anderson, Reagan and President Carter in the toughest language he has used in his entire campaign.

Bush has spent about $350,000 in Connecticut, compared with $150,000 for Reagan and $80,000 for Anderson.

Gardner, the Hartford councilman running the Anderson campaign, said in a state which is historically moderate, after this kind of effort, would indicate that Bush can't sell anywhere.This could be his last hurrah."

But reagan has attracted support here, not only from traditionally conservative Republicans and the strong right-to-life groups, but from state Sen. Nancy Lee Johnson of New Britain, a feminist and leading GOP liberal.

Reagan's state campaign coordinator, Tony Nania, cut his political teeth as a liberal activist for Eugene J. McCarthy and George McGovern and took time out from the Reagan effort last week to attend memorial services in New York for his one-time mentor, Allard K. Lowenstein.

Johnson and Nania said they were attracted to Reagan by what Johnson called his "moderate record" as governor of California, and by the belief that he could unite the Republican Party to cut back government spending and bureaucracy, as both have come to believe the public is demanding. s

In New York, Reagan has picked up similar pragmatic support from the Republican leaders of Brooklyn, the Bronx, Queens, Staten Island, Suffolk and Nassau counties -- partially because his views no longer seem outside the GOP mainstream and partly because he has the look of a winner. "These guys have no place else to go," commented Roger Stone, a Connecticut native and member of Reagan's national staff.

In Connecticut, Nania and Stone do not hedge their predictions of a Reagan victory.

Bush, however, has been telling crowds across the state that "we are going to do well here and turn this fight around." But Helen Robbins, the Connecticut coordinator for Bush, said, "We've got a good organization, but we may be the victims of circumstances. After New Hampshire, there is quite a letdown. People in Connecticut don't dislike George, but there may be a sense of hopelessness about the campaign that keeps the undecided from making the decision that it's worthwhile trying to save him."

In the Democratic race in New York state, a New York Daily News poll published in Monday's editions showed Carter leading Kennedy by 56 percent to 36 percent in the state, a lead 7 points smaller than Carter had in a similar poll last week.