The Senate is moving toward approval of a constitutional amendment to require balanced budgets only a month after sanctioning a budget with back-to-back deficits of more than $100 billion for this year and next year.

The House is preparing to debate, and possibly approve, a resolution calling for a freeze on U.S. and Soviet nuclear weapons even as it gets ready to launch the second phase of the biggest peacetime military buildup in history.

In both houses, the latest fad is a flat-rate tax to wipe the slate clean of tax loopholes that Congress keeps enacting, including a mother lode of them that was approved only last year.

Sen. Charles McC. Mathias Jr. (R-Md.) has called the balanced-budget amendment "a fig leaf to cover our embarrassment" over soaring deficits, and the same has been said in more prosaic ways of the other crusades that are grabbing the attention of the 97th Congress on its downhill slide toward the Nov. 2 elections.

But these assorted endeavors, cutting across both political and ideological lines, may be more than just bet-hedging contradictions or passing gestures of political convenience in an election year.

They reflect and underscore the frustrations that many lawmakers, responding to constituent pressures, feel about the government's inability to address people's deepest concerns about public policy--frustrations that sometimes turn into desperation for politicians as Election Day approaches.

In some cases these causes offer a quick-fix or a grand gesture that promises more in the future than the government can deliver now or maybe even later--a "cop-out," some critics say.

But a common thread running throughout is a gnawing sensation among many members of Congress that their constituents are fed up with the complexities--and the politicians who keep talking about them--that stand in the way of getting what they want for themselves and their country.

"There's a very strong feeling that if you leave something important to your elected leaders they're bound to muck it up," observed Rep. Thomas J. Downey (D-N.Y.).

One of the main arguments for the balanced-budget amendment is that Congress has so amply demonstrated a built-in bias for spending and deficits that it can only be restrained by the Constitution itself.

The essence of the nuclear freeze argument is to cut through all the political and diplomatic fog and "stop the arms race by simply stopping it," said Rep. Jonathan B. Bingham (D-N.Y.), one of its leading proponents.

As for the flat-rate tax, even many of those who criticize it as potentially inequitable acknowledge that the only way to reform and simplify the tax code may be with a clean sweep.

The coalitions behind these movements differ widely but the movements share at least three main elements: They address problems people care about a lot, they appear simple and direct, and they embody a wholesome, catchy type of symbolism. Whether they would achieve the desired result is a matter of debate.

"We're dealing with the politics of symbolism and simple solutions," said Rep. Les Aspin (D-Wis.), who has misgivings about how each of the causes would turn out but acknowledges the political appeal of these crash-through-the-barriers attacks on major national problems.

"People want litmus tests and answers they can understand. And there's a kind of nice anti-bureaucratic, or anti-expert, flair" to solutions that short-circuit those who know so much about an issue that they see its flaws as well as its promises, Aspin said last week.

"People have a feeling the experts keep them out of the action, and this cuts them in. People are saying let's cut all the falderal and gibberish and get things done. It's a call for action."

From the other side of the political spectrum, Rep. Ed Bethune (R-Ark.) agrees that Congress is increasingly tempted by simple, sweeping solutions.

"It's a natural phenomenon but it may be exaggerated now because of all the volatility and swings of philosophy out there" as well as the confused nature of political control in Congress, said Bethune, who is caught up in the maelstrom on the balanced-budget amendment.

A staunch fiscal conservative, Bethune wants a balanced budget. But he says he has always resisted the temptation of constitutional amendments as solutions to political problems and doesn't want to give in now.

"What I'm trying to do is fortify myself against an impulse to go along with a very simple course to demonstrate my fondness for a balanced budget . . . but I've got to look hard at it because of the drumbeat of support out there for it," he said. He is, Bethune added, in "anguish and disarray."

One reason why Congress may be reaching for bold, symbolic gestures is that President Reagan has been so good at it, sometimes at Congress' own expense, some members concede.

"Unfortunately, the modern, complex world requires that problems be solved in a complex way," Downey said. "The average American's attention span is short anyway, and it's been made shorter by television. Now it's exacerbated by a president who has simple nostrums for almost everything."

Not only is Congress responding to Reagan, but also, "There's a larger problem of people wanting action on terribly complicated problems without being willing to sift through them for the answers," Downey said.

Aspin said: "The difficulty is in keeping the solution from being so simple that it becomes a straitjacket."

Along with other arms control advocates, Aspin has been negotiating for modifying amendments to the nuclear freeze proposal, just as Senate Budget Committee Chairman Pete V. Domenici (R-N.M.), a deficit foe if there ever was one, has been doing in the case of the balanced-budget amendment.

A debate earlier this month on the budget amendment illustrated as well as anything why Congress, sick to death over trying and failing to get a handle on the deficit, is grasping for the sweeping solution and why some lawmakers are concerned that it may be a hollow promise.

"This spending bias of Congress has yet to be corrected by internal reform, even though a majority of the members of Congress believe that large deficits and excessive government spending severely damage the economy," said Sen. Howell Heflin (D-Ala.), a backer of the amendment.

Congress wouldn't be facing the constitutional issue at all "if we had met the tough problems that faced us head-on--what to do with the automatic cost-of-living increases for Social Security, the increases in defense spending, the indexation of the tax brackets . . . ," said Mathias, who advocates simple legislation to require a balanced budget instead of writing it into the Constitution.

That brought Sen. Orrin G. Hatch (R-Utah), chief sponsor of the amendment, to his feet in indignation, asking "how anybody after watching . . . 20 of the last 21 years produce unbalanced budgets can come in and piously say that a statute would solve our problems."

History shows it probably wouldn't. In fact, there is already a law on the books requiring a balanced budget. But history also indicates that even the Constitution cannot totally protect the country and its government.

"No legal formula," noted Mathias, "will be adequate to save us from ourselves."