The quiet ascension of Rep. Richard A. Gephardt to the chairmanship of the 253-member House Democratic Caucus, an official step onto the House leadership ladder, is in some ways a rite of passage for the political generation of Democrats coming of age.

Gephardt, the five-term congressman from predominantly white, heavily Roman Catholic south St. Louis, is best known to the public as the coauthor of one of the modified flat tax proposals under consideration on the Hill.

But in the House he is a symbol of much more -- the "new-breed" House member. First elected in the last 10 years, this group occupies more than 60 percent of the Democratic seats in the House. As a group they are less devoted to New Deal or Great Society programs than are their party elders. They are convinced of the need for "new ideas" but groping for what those ideas are.

The bruising defeat of the Democrat nominee in the last two presidential elections and the announced retirement in 1986 of House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) have spurred demands by these younger Democrats for more say in the direction of the House and the party.

While uncertain and often divided about where they are going, these newer House Democrats are clear on one point: On the list of those likely to lead, Gephardt figures prominently.

Conservatively dressed, his neatly cropped red hair carefully in place and his voice well-modulated, he is the perfect television-age candidate. At 43, Gephardt seems like the Eagle Scout he once was.

He is intelligent and articulate. And, like the other coauthor of the tax-revision bill, Sen. Bill Bradley (D-N.J.), he is known as a "technocrat" who helps develop "new ideas." Yet, he also is viewed as one of the most politically adept coalition builders in the House.

But he lacks the strong appeal that a more traditional liberal, such as Sen. Edward M. Kennedy (D-Mass.), has for many groups in the old Democratic coalition.

"His greatest strength is his ability to sit in a room until everyone else is bored silly and then put forward his compromise and thrash it out," said a longtime associate. "The question for Gephardt -- and all these younger guys -- is does he bleed."

Gephardt's moderate evolutionary approach, combined with a knack for dogged work, has made him a favorite of the House Democratic leadership. This has allowed him to act as a bridge from the leaders to the newer House members.

This fall, for instance, Gephardt brought together a group of younger Democrats frustrated by their lack of influence with the leadership. He made sure the meetings did not take O'Neill by surprise and helped persuade the speaker to open the leadership process with more caucus meetings and the creation of a speaker's cabinet, or executive committee, to set direction for House Democrats.

Gephardt is expected to play a similarly conciliatory role in the caucus, the organization of all House Democrats, which O'Neill has said will meet more frequently to allow greater communication on legislation, agendas and policy between the leadership and House factions.

Unlike the "Watergate babies" elected in 1974, who challenged the Democratic rules, procedures and committee chairmen of a then-rigidly run House, Gephardt and other young Democrats have been team players, in part because they entered a more democratized House.

Like other "new-breed" Democrats, who went into politics during a time of high inflation and mounting federal deficits, Gephardt has focused on economic issues.

From his seats on the House Budget Committee and the Ways and Means Committee, he helped develop the Bradley-Gephardt tax proposal, which is designed to simplify the tax code by eliminating most deductions and replacing the present 14 brackets with three. And he proposed alternatives to President Reagan's budgets and an approach to reducing health costs that relies on the marketplace as much as government control.

Even before Reagan was elected in 1980, promising cuts in federal spending, Gephardt and four other "new breed" Budget Committee Democrats were searching the budget line-by-line for wasteful programs, an effort the leadership did not support.

Gephardt also played a key role in drafting an economic "blueprint" for the future, published by the House Democratic Caucus in 1982. This paper was in large part a high-technology manifesto for a new industrial policy. It was embraced by many of the new breed but, to the frustration of many of them, never drew much interest from House leaders.

It recommended that government and private incentives be used to bring about a "technological" revolution and the modernizing of old industries. This, not new public-works or public-employment programs, would restore the nation's long-term economic health, the blueprint said.

Gephardt's skepticism about traditional Democratic approaches is based on his experience in local government when cities were decaying despite Great Society programs.

As an alderman, he felt that the old Model Cities program did little to reverse St. Louis' downward spiral. But he said the city began to revive somewhat when he and other "young turks" elected to the board in the early 1970s supplemented federal and state aid with tax incentives to keep businesses in the city and encouraged neighborhood groups to patrol, clean and help develop the city.

Gephardt said many older lawmakers feel strongly about these Great Society programs because they fought to create them. But many of the younger generation of House Democrats saw how the programs operated on a local level.

"You come in with a questioning of some of the underlying assumptions. It's just a question of pragmatism, what works, what doesn't work, especially in times of budget problems," he said.

The result, he said, is "an influx of a lot of new members in the House with different ideas, different concerns and a little different view of government."

Gephardt ran afoul of senior citizens when he suggested that middle-class programs, such as Social Security, should be scrutinized along with programs for the poor. And his social conservatism has caused him problems with liberals, women and blacks.

Gephardt opposes abortion, school busing and gun control and has supported school prayer and tuition tax credits for parents who have children in private schools.

Gephardt also voted against extending the deadline for ratification of the Equal Rights Amendment. Last year he lead the unsuccessful House Democratic leadership effort to pass the Equal Rights Amendment again, but that has not quelled concern about him among feminist groups.

According to one of the more liberal young House Democrats, Gephardt's social conservatism may be the one block to his seemingly inevitable political rise in the House.

He is mentioned as a potential candidate for speaker or majority leader in 1986. But, with typical caution, Gephardt is keeping his plans to himself. "I really, honestly, don't have a game plan or agenda," he said. "I'm interested in working hard as caucus chairman, and we'll see what happens after that."