About 14 percent of Americans are entrenched in poverty, setting a "plateau that is higher than poverty levels have been in this country in nearly 20 years," according to an analysis released yesterday by a group called Thanksgiving Action on Poverty.

The analysis also said middle-class Americans are receiving a smaller share of total income and have become more vulnerable to poverty. One of four Americans slips below the poverty level every 10 years, and one of three Americans is currently within twice the poverty level, it said.

In 1984, a family of four with cash income of less than $10,610 was considered impoverished by government standards, as was an individual with cash income of less than $5,278.

"The middle-class is no longer secure," the group said in issuing a "call to action" against poverty. The group is composed of 35 organizations representing a wide range of political, social and religious views.

"Many analysts see a 'shrinking middle' as declining industries are replaced by lower-wage service jobs. Young households now require the incomes of two earners for the same standard of living their parents achieved with one earner. Most Americans are only a paycheck or an illness or a spouse away from poverty," it said.

The analysis, performed by the Center on Budget and Policy Priorities, also said full-time work no longer ensures against poverty since 2 million Americans work full-time "at or slightly above the minimum wage -- stagnant at $3.35 an hour since 1981," yet remain in poverty.

The analysis acknowledged that the poverty rate dropped from 15.2 percent in 1983 to 14.4 percent in 1984. But it said the rate is at its highest since 1966, except for "the recession years of 1982 and 1983."

It also noted that 4.4 million more persons are classified as poor than in 1980 and 8.3 million more than in 1970.

"While the poverty rate dropped a bit in 1984," it said, "no significant drop is expected in 1985. The modest improvement in 1984 was due to a major decline in unemployment.

"But unemployment has stayed in the 7 to 7.5 percent range for all of 1985. Poverty may very well be stuck at a new plateau for some years to come -- a plateau that is higher than poverty levels have been in this country in nearly 20 years," it added.

Dr. Arthur S. Flemming, former secretary of health, education and welfare, led a news conference at which the report was released yesterday.

He was joined by Brother Joseph Berg of the National Conference of Catholic Charities, Haviland Houston of the National Council of Churches, Rabbi David Saperstein of the Union of American Hebrew Congregations, Robert Greenstein of the Center on Budget and Policy Priorities and Donna L. Brazile of the National Political Congress of Black Women.

Brazile is to be national coordinator of "Thanksgiving Action on Poverty" meetings in five cities next weekend.

"At a time of public thanks and private giving," the group said, "we believe that private charity is not enough, either to deal with poverty or to alleviate the economic stress of which poverty is the most extreme evidence. At stake is nothing less than the fiber of our society and the future of our nation . . . .

"More than 33 milion Americans now eke out an existence below a bare-bones standard of living," the group said.

"Millions more livein constant risk of poverty's revolving door' . . . in the economy as a whole, the gaps are widening, th rich are getting richer, the poor are getting poorer. Income inequality is greater now than at any time since the statistic has been measured - it is measuring the American dream."

The group's weekend protestsare scheduled in Atlanta, Des Moines, New York, Chicago and Los Angeles to begin a year-long effort to respond to poverty, Flemming said.