When the House last month killed a $500 billion deficit-reduction package in large part because rank-and-file lawmakers feared an angry reaction from elderly voters to the measure's cuts in Medicare, it provided the latest illustration of a puzzling phenomenon: In an era when House members have become almost immune to successful electoral challenge, they seem to many observers -- including many lawmakers themselves -- to have become more averse to taking risks.

In this view, the astonishingly high reelection rate of House incumbents in recent elections -- 98 percent in 1986 and 98.5 percent in 1988 -- has done little or nothing to embolden lawmakers to make the hard choices facing them. In fact, say some lawmakers, the opposite has occurred: The safer the House gets, the more timid it becomes.

"It's counter-intuitive," observed Rep. Thomas J. Downey (D-N.Y.). "You would think the more secure you are the more of a risk-taker you would be. But it's the reverse. . . . Sometimes the longer you are here, the less capable you are of determining what you stand for."

"We're partly to blame," observed Minority Leader Robert H. Michel (R-Ill.). He noted that Congress -- as well as two Republican presidents -- in the past decade encouraged the electorate to believe it can have low taxes and a high level of government services and never have to sacrifice to trim a soaring deficit. "We've led people on to think they can have their cake and eat it too," said Michel.

Some political strategists suggest there is good reason for incumbents to be nervous about their reelection chances this year, when voters across the country are expressing to pollsters their general disgust with government and Congress, an attitude heightened by the institution's fumbling on the budget.

Neil Newhouse, a GOP consultant who advises House candidates, predicts many incumbents will see their reelection margins fall on Tuesday, and that some who have not dealt aggressively with anti-incumbent sentiment will lose. Newhouse and others say that as many as 15 House members could lose their seats -- about twice the number of two years ago, but still a reelection rate of about 96 percent.

Even normally safe House incumbents have spent the week since Congress adjourned furiously trying to beef up their support at home and nervously watching unusually volatile poll numbers.

But even with the uncertainties of the 1990 election, many members of Congress are disgusted with their colleagues' caution, particularly on the issue of reducing a budget deficit.

Asked what Congress's performance on the deficit this year says about its collective political will, the normally low-key Michel exploded in frustration: "Not very damn much. That to me is the most distressing, discouraging, gnawing thing. I get home at night and wonder, 'Why can't we do things in the best interest of the country?' "

To be sure, the House during the 101st Congress confronted some politically tough votes. It voted to raise its own pay, beat back a constitutional amendment to ban flag burning, repelled attempts to gut the National Endowment for the Arts because of a controversy over obscene art and ultimately agreed to cut the deficit by $492 billion over five years.

But more typical, say internal critics of the House, was a May 24 vote overwhelmingly rejecting legislation to pay for a $25 million overrun in House mail costs. Though a "no" vote was entirely symbolic -- under law the Postal Service must deliver congressionally franked mail even if there is no money appropriated to cover it -- the measure was defeated 208 to 161 because lawmakers were apparently afraid to cast a vote for franked mail in an election year.

Election season dynamics were also at work more recently when the House overwhelmingly rejected a recommendation by the Appropriations Committee to raise the pay of congressional staff. Among those voting against the increase were 136 House members who last year -- well before the election -- voted to raise their own pay.

But the most glaring example of congressional timidity in the 101st Congress came last year when the House voted 360 to 66 to repeal legislation providing Medicare beneficiaries insurance against catastrophic illness. The abrupt about-face -- a year earlier the House approved this major expansion of Medicare 328 to 72 -- followed an extraordinary public protest by senior citizens and groups representing them even though only about 40 percent of the 33 million beneficiaries would have had to pay even a portion of the supplemental premium to cover the cost of the new benefits.

"In Oklahoma, for 80 percent of seniors it was a better deal," said Rep. Mike Synar (D-Okla.). "But because of the fear of the 20 percent who were constantly yelling, we flipped the system."

House members point to a number of institutional and political reasons for their collective caution. They include the breakdown in party discipline since the 1970s as the leadership lost power and younger members pushed for reforms that have made them independent contractors both as legislators and as candidates; a changing campaign climate in which opponents have become ever more adept at zeroing in on two or three votes and using them for 30-second negative television spots; a shrinking electorate that has given more weight to blocs of single-issue constituencies; and a concerted effort by conservative Republicans to force votes on tough values-oriented issues they believe will undermine Democratic control of the House.

"People here really have to work very hard to be in effect insulated from the kind of political terrorism that can take place," said Rep. Vic Fazio (D-Calif.). "It's the political terrorism of those 30-second spots. It makes people feel very vulnerable."

That shared feeling of vulnerability binds House incumbents together, sometimes even across party lines. Last month, for example, a Democratic committee chairman in the House, Rep. Henry B. Gonzalez (D-Tex.), cut a reelection commercial for a Republican colleague, Rep. Chalmers P. Wylie (R-Ohio), who serves as the ranking GOP member of Gonzalez's banking panel.

House members don't have to look far to find an example of what can happen to an incumbent and how a loss affects behavior.

Sixteen years ago, Rep. Bob Carr (D-Mich.) swept into Washington with the Watergate class of 1974 and immediately became one of the most rambunctious members of that fabled class of rebels. He called for the resignation of Speaker Carl Albert (D-Okla.) and frequently bucked the committee system to press his positions against major military systems.

But six years after he arrived, Carr was defeated in the 1980 Reagan landslide. Reelected two years later, Carr, according to his colleagues, returned as only a shadow of his former self.

"He came back in '82 and basically said, 'I'm not going to do anything to get in trouble,' " said one colleague.

Carr rejects that criticism as "a conventional story line" and says his voting record in recent years merely reflects a moderate district that wants spending and taxes cut. "I think my job is to reflect their attitudes," said Carr. "I study my constituency."

Carr is certainly not alone in being cautious when voting on fiscal policy. He and 66 of his colleagues share an unusual distinction: After voting earlier this year for a constitutional amendment to require a balanced federal budget, they voted against both the summit agreement to cut the deficit by $500 billion and the replacement budget that then passed the House. Members of this group also have either no electoral opposition this year or an opponent who has raised less than $25,000 in campaign contributions.

Acknowledging that House members have grown more cautious over the years, Rep. Henry J. Hyde (R-Ill.) argued that the trend is not necessarily pernicious.

"More thoughtful voting is important," he said. "If people think more about how they are voting, we'll get better government."

House members who frequently cast tough votes lament the timidity of the institution.

Rep. Fred Grandy (R-Iowa), now in his second term, has often taken stands unpopular in his district, from supporting contra aid to opposing the flag amendment and supporting the budget summit agreement.

"To me the only thing that makes this job interesting is the risk potential, when you have to put it on the line," said Grandy, a former actor. "If I'd wanted to stay risk-free I would have stayed in Hollywood."

Ultimately, Grandy said, voters respect a politician with the guts to vote his beliefs. "Everything I get from back home is 'make a decision, you got the pay raise, do something,' " he said.

Though Congress isn't providing it, said Grandy, "there is an enormous market for governing out there."