Back in December, 174 Democrats in the HOuse of Representatives voted for a package of Social Security tax increases designed to pump an additional $227 billion into the depleted trust funds over the next 10 years.

Now, more than 150 of those Democrats have signed a petition for a party caucus, after the Easter recess, to roll back those increases and, in stead, finance a substantial portion of the burden from general revenues.

Such a rapid reversal of major policy is rare - if not unprecedented - and quizzical senior members of congress are inclihed to think it says more about the changes in Comgress than it does about changes in the country.

Rep. Barber B. Conable Jr. (N.Y.), who led the House Republicans in their near-animous opposition to the payroll tax boosts last December, commented the other day: "All these fresh young guys who have never had to vote for anything unpopular are in full flight."

One of those "young guys," Rep. Toby Moffett (D), a 33-year-old second-termer from Connecticut, said, "The worst that can be said is that we're knee-jerk and just reacting to pressure. The best is that we're in touch, and we're hearing from people who are the active, vocal ones in our districts."

Whichever view one accepts, members of Congress, the White House staff and interest groups are viewing the Social Security phenomenon as an intriguing indicator of the changing character of the legislative branch.

The 95th Congress is earning a reputation as a quirky, unpredictable and, in some respects, unmanageable collection of political mavericks.

From the surprise rejection of a common situs labor bill early in 1977 to last week's successful floor revolts against leadership-backed education aid and election-financed laws, the House, in particular, has delighted in defying predictions.

Many members of Congress and close students of the institution link this unpredictability to the rapid change in membership in recent years.

Over half the Democrats in the House - 150 of 288 - have served fewer than three terms. They feel less secure in their home districts than those who have been through more campaigns.

Many of those younger men and women were stirred to political activism by their opposition to the Vietnam war or their personal antipathy to Richard Nixon. But they have little if any commitment to traditional Democratic economic policies, and they are less dependent on labor unions, whose leaders try to enforce that welfare state philosophy.

The newer Democrats tend to have their own personal politic organizations. For many who represent suburban districts that before Watergate were regarded as safely Republican, there is little Democratic party organization on which they can rely.

All this, in the view of Capitol Hill veterans, makes them prone to react - or overreact - to shifts of opinion among their constituents.

As William Cable, President Carter's liasion man with the House and previously a House staff member, said, "The younger members are very sensitive to the pressures they receives from their districts. Sometimes they react even to pressures that aren't there. But my Bible says that perceptions are more important than realities on the Hill."

Cable said polls taken for the White House by Patrick Caddell and some union-sponsored polls of marginal comgressional districts show "70 percent of the people either don't know or don't care about the changes in Social Security taxes."

Spot checks with members of the House find a very mixed pattern of pressure.

Butler C. Derrick Jr. (D S.C.), a third-termer, said he has had "quite a bit" of complaint, mainly from two groups: prospering young professional and businessmen, who will feel the effects of the rise in the taxable base from $17,700 to $42,000, and textile worker families, where both husband and wife work to achieve a $15,000-$20,000 family income and thus are subjected to double payroll taxation on a rather modest income.

Derrisk said he would "seriously consider" softening the blow by "putting all or a portion of the Medicare costs into general revenues."

On the other hand, James J. Blanchard (D-Mich.), a second-termer, and Norman D. Dicks (D-Wash.), a freshman, who joined Derrick and most other Democrats in voting for the higher payroll taxes last December, say it has not been much of an issue in their districts.

"I can't say I'm getting that much heat," said Blanchard. "Last year, there was enormous concern the (trust) fund would go broke. Now, naturally, thw people who are paying the bill and will someday receive benefits are expressing some concern.But I'm not sure Congress is not overreacting."

Dick said he has "not had a lot of comment on it, and the White House" - which is resisting any tinkering with the tax this year - "may be right in saying we'll cause ourselves more grief politically by going back to the issue."

Yet, both Blanchard and Dicks have signed the petition calling for a party caucus to reconsider the Social Security tax boosts.

Blanchard, while saying "we would look like cowards and fools to completely backtrack" on last December's action, would nonetheless like to see some of the burden shifted to income taxes because "that is a more progressive way."

And Dicks said, "I think a lot of members feel we made a mistake. We should have listened to the administration and used some general revenues to balance this thing off," as Carter originally advocated in his 1977 proposal.

The turnabout leaves some older members bemused. Rep. Sam Gibbons (D-Fla.), a veteran member of Ways and Means who opposed the payroll tax hike last December, said, "I didn't vote for the last one and I tried to tell these fellas that they shouldn't either . . . The payroll tax has gone much too high, I told them . . . I've seen it coming for sometime, and I am damned if I know why they couldn't."

Andrew Jacobs Jr. (D-Ind.), another Ways and Means members who opposed the payroll boosts last year, recalled that his father, also a congressman, like to say, "You can get people to taste the pudding, but you can't get them to read the recipe."

"They've tasted the pudding now," the younger Jacobs said switching metaphors in midstream, "and now they want to fire the retrorockets."

"What they are proposing to do scares the hell out of me," he continued. "They want to borrow to pay Social Security. When you have a deficit, that's what it means, when you talk about financing Social Security out of general revenues."

Most members of the House say privately there is no good solution to the Social Security financing problem - none, that is, without political problems.

But Republican Conable sees the strong reaction among the Democrats as a measure of the changed membership.

"In the old days," he said, "when Congress was criticized for being 'out of touch,' you did have a lot of very senior people who were relatively remote and unassailable. When a controversy blew up, they would just hunker down and wait for it to blow over . . . These fellows, just a slight shift in the breeze sets them moving."

Moffett, a leader among the bigclass of Democrats elected in the Watergate year of 1974, expressed much the same thought.

"Before our class arrived, there were maybe four or five Democrats who had regular open forums in their districts. Now, there must be 80 or 90 of us who do that, as a way of staying close to our people," Moffett said.

Moffett's pulse-takings have yielded some surprises.

"We're not hearing from the old folks at our forums. We thought they'd be grateful that we had secured the trust fund so their benefits would be assured. But they are so cynical about government, as everyone is, that they don't believe we've really taken care of their problems.

"And the workers are really cussing us out for raising their taxes. I had a meeting with 150 insurance company employes and their wives and I thought they were going to tear me apart . . . "

The protest that are heard are part of what many in Congress are calling the middle-class tax revolt. The same force fuels the effort to provide tuition tax-credits for camilies that send their children to private schools.

To older Democratic liberals, like Richard Brolling (Mo.), such moves are a betrayal of the party tradition. "There's a great feeling up here now," ha said, "that we just ought to abandon the poor, because inflation is the only issue that counts anyway."

"That's okay," he said, "unless you have a feeling for social justice. And we don't have a party that has that today . . . I think a lot of the newer members are from upper-middle class familied themselves. They have an entirely diffrent reaction to these issues than those of us who grow up in the Deparssion.

"And Cable, the White House lobbyist, made another observation that may be pertinent.

"The people that are going to be hurt by the Social Security increase are the people most members of Congress play cards or golf with - their buddies, the bankers and lawyers and brokers. I don't suppouse too many members of Congress sit around in the evening playing bridge with auto workers. They're more inclined to meet the 5 percent who make more than $35,000 than the 95 percent who make less. And they know raising the base on Social Security tax earning is going to take a bite out of them. "We're finally making the Social Security tax slightly progressive, and progressivity is taking a toll out of those who have very good access to decision-makers.