GEORGE BUSH has been enjoying the most protracted honeymoon in the modern history of the presidency. Too bad for him, but it's over.

The president's political fortunes will wax and wane many times before he again must face the voters. But for the first time, a faint aura of vulnerability has attached itself to Bush, and it could affect his ability to deal with the vexing domestic issues he now confronts.

On the budget-deficit front, the president has nothing but unpalatable options in his summit with Congress. He must also deal with the savings and loan debacle at the very moment outraged citizens are waking up to the cost and scope of the catastrophe. His minimalist approach to governing isn't likely to wear well in the event of an economic downturn (which a near-majority of the electorate now fears) or among voters who are starting to take a sober look back at the upper-income consumption binge of the 1980s and an anxious look forward to the stored-up problems for the 1990s. And Bush's establishment persona leaves him ill-equipped to pull off the political masterstroke that his predecessor handled so effortlessly: In an anti-government age, a president must keep fanning the flames of populist resentment against the very government he heads.

These incipient signs of trouble and hints of weakness for Bush have already begun to embolden congressional Democrats. They defied the president on the flag-desecration amendment last week (proving they're no longer spooked by the 1988 campaign); they're starting to hector him about the S&Ls (last week, the White House hectored back); and they show no sign of giving quarter in the budget negotiations that are likely to come to an anti-climax this summer or fall.

Perhaps more important for the long term, the outlines of the Democratic case against Bush in 1992 are now in place. It will seek to ignite latent middle-class anger at the alleged coddle-the-rich-and-rob-the-future policies of the Reagan-Bush era, and it will call for national economic renewal and reinvestment of the sort that can only be managed by a bold leader. These twin appeals -- one negative, the other affirmative -- will be glued together with heavy doses of populist Washington-bashing.

To be sure, the Democrats don't yet have a candidate, and they're facing some daunting history. For two centuries, the rule has been simple: Incumbent presidents rarely get beaten, except for cause. "Unless we have some economic or foreign policy catastrophe," said GOP consultant Craig Shirley, "Bush's reelection will be a slam-dunk."

That's been the received wisdom for the past year and a half as Bush's stewardship of foreign policy during the demise of communism has earned him record high voter approval ratings. Now, however, with nettlesome new domestic issues dominating the landscape, Bush's continued high approval ratings belie mounting vulnerabilities. Among them:

A recent survey taken by GOP pollster Richard Wirthlin found the "largest confidence disparity we have ever tested" between a president's approval rating (71 percent) and the number of people who think the country is heading in the right direction (36 percent). The same poll also found that only 22 percent of voters express "strong approval" for Bush -- which means that while he continues to enjoy an overall approval rating higher than Reagan's, his core support is much lower. "What all this says to me," said one GOP pollster who asked not to be identified, "is that one or two months of bad economic news, and Bush's rating will start sinking very fast."

Sixty percent of voters think the country is "off on the wrong track" (the most pessimistic reading in 2 1/2 years) and while 46 percent think the economy is getting worse, just 7 percent think it is getting better, according to a Washington Post/ABC survey. The same survey found that Bush's ratings for handling the economy have slipped to the lowest levels of his presidency, with just a small majority (52-45 percent) now expressing approval.

At a time when many Americans believe that the nation may be losing control of its economic future, Bush is increasingly viewed as a reactive, caretaker president. By 53-34 percent, according to survey taken by Democratic pollsters Harrison Hickman and Paul Maslin, voters think he "mainly deals with today's problems" rather than "gets the country ready for tomorrow." By 50-37 percent, they say he "only takes action after problems have developed." This may or may not be a vulnerability. As Democratic pollster Thomas Kiley noted: "Voters are fearful of the difficult choices they know the nation faces, and there is a kind of avoidance going on. In some ways, Bush is a symbol of that avoidance -- he's a reflection of the voters' confusion about the future. You can make an argument that this didn't hurt him in 1988; it actually helped him. The question is whether there will be a more activist mood by 1992."

As the budget deficit numbers keep growing -- Budget Director Richard Darman has raised his deficit projection for fiscal 1991 to $159 billion (up from $100.5 billion in January) -- Bush's maneuvering room keeps shrinking. "If he raises taxes, he's gone back on his word and given up a key Republican issue," said Democratic pollster Mark Mellman. "If he doesn't, he'll propose a small solution to what he has defined as a big problem. That goes to his vulnerability on leadership." Democratic budget negotiators have shown little interest in helping the president out of this ditch. "He dug it," said one Democratic congressional staffer. "Let him climb out of it."

The White House has gotten testy about the S&L scandal, with Press Secretary Marlin Fitzwater last week launching an unusually personal attack on the administration's Democratic critics. "These guys are unraveling," said Brad Johnson, the top Washington aide to New York's Democratic governor, Gov. Mario M. Cuomo, who has been hammering at the S&L issue for a year. "They can sense the class anger that's out there -- they can see that this is a story of greed and deregulation -- two classic weakness of Republican administrations."

Bush's attempt to inoculate his administration against charges that it has been slow to prosecute S&L crooks -- laid out in Friday's press conference at which he called for doubling funding for fraud investigations -- is fraught with peril. Legal experts say cases will be difficult to win, and voters are intuitively skeptical about the enthusiasm of Republican administrations for going after white-collar criminals.

Closer to home, Bush's son, Neil, is under fire for failing to disclose loans his oil exploration business received from a developer in arrears on $11 million in loans and other obligations to a Denver S&L on whose board Bush served. (The S&L, Silverado, subsequently failed, costing taxpayers $1 billion.)

"This {the Neil Bush case} is the heart of the S&L crisis," New York Daily News columnist Lars-Erik Nelson wrote last week. "It is not outright embezzlement; rather it is a small group of friends who found a wonderful way to loot the Treasury . . . . You take other people's money out of a bank and loan it to each other." Asked Nelson, "If you lay off Neil Bush, whom can you go after?"

Fitzwater's partisan threats and edgy defense of Neil Bush last week left many Democrats speculating that all the White House had accomplished was to call more attention to the issue. "What Fitzwater did was turn Neil Bush into Billy Carter," said one Democratic congressional staffer. "He hung a 'kick me' sign on the president's son."

Some of the inherent contradictions of Bush's campaign rhetoric are starting to pinch. He has threatened to veto legislation that would force employers to give employees unpaid leaves for pregnancies or for illnesses of family members. The business community and conservative think tanks are lobbying heavily against the proposal -- sizing it up as a nose-under-the-tent for a raft of new "mandated benefits" that many Democrats in Congress would like to impose on employers. But women's groups are lobbying equally hard for it, reminding Bush what the "kinder-and-gentler" candidate said during the 1988 campaign: "We need to assure that women don't have to worry about getting their jobs back after having a child or caring for a child during a serious illness."

The Supreme Court decision last summer upholding the rights of states to restrict abortions has activated a heretofore complacent abortion-rights majority in the electorate. As election results from 1989 and 1990 already attest, abortion-rights candidates are now advantaged -- leaving Bush, politically, on the wrong side of the issue.

Many Republican strategists acknowledge that the remainder of Bush's term won't be the speed-boat romp it's been so far, but none seems inordinately concerned.

"This far into any administration, the halo is bound to dim somewhat, especially when you've got some nervousness about the economy," said Charles Black, a leading GOP consultant. "But the fact is that George Bush has built up a huge amount of goodwill and personal credibility. And you're certainly not going to beat him on foreign policy."

Black said it would take a "full national recession" for Bush to be in jeopardy in 1992. While that asssessment is widely shared by analysts in both parties, it's not unanimous. "I have never believed that you need an economic collapse for George Bush to be vulnerable," said Cuomo's Johnson. "By 1992, people are going to expect a peace dividend. They're bound to be frustrated that while our philosophies are being embraced around the world, and while we should be enjoying a renaissance here at home, we're bankrupt instead. And when you tell them that the reason there isn't a peace dividend is that it was stolen by Republican policies and scandals, you're going to strike a nerve."

Many Democrats also feel their 1992 case will be bolstered by the line of analysis advanced by Republican strategist Kevin Phillips in a just-published book, "The Politics of Rich and Poor: Wealth and the American Electorate in the Reagan Aftermath."

Phillips notes that in the decade since Reagan was elected, private-sector chief executive officers saw their incomes grow by 50 percent in inflation-adjusted dollars while workers saw theirs fall by 5 percent. He writes that the overall tax burden on the rich has decreased, while it has held even or risen for virtually everyone else. There has been no comparable upsurge in the relative well-being of the upper class, he argues, since the Robber Baron era of the late 19th century. "Money politics -- be it the avarice of the financiers or the question of who will pay for the binges of the '80s -- is shaping up as a prime theme for the 1990s," Phillips writes.

Along a parallel track, Democrats also believe that a case can be made against the Reagan-Bush era for its disinvestment in public infrastructure, in research and development, in education and job training and in other long-term strategies designed to keep the nation competitive in an increasingly globalized economy. Polls show that most middle-class Americans worry that their children won't find it easy to buy a house or pay for a college education -- and these fears of a declining standard-of-living dovetail with the robbing-the-future indictment of Republican policies.

"Over the past two decades, the genius of the Republicans has been to forge a perceptual alliance of the middle-class and the rich against the poor, based primarily on cultural, foreign policy and anti-government themes," said William Galston, a University of Maryland political scientist and sometime Democratic adviser. "The challenge for the Democrats is to create a new perceptual alliance of the middle-class and the poor against the rich, based primarily on economic anxiety."

The challenge for George Bush will be to demonstrate that the success of his presidency is built on more than just the good fortune of being on duty when communism collapsed.