A case for informed, moral, responsive politics; Speaking Up, by Millicent Fenwick. New York: Harper & Row. 187 pp. $12.95.

Republicans weary of having their party maligned as callous and unthinking will welcome this book. So will all those - of whatever political stripe - who value compassion, integrity, balance, and directness.

New Jersey's Millicent Fenwick, who has been a member of the US House of Representatives since 1975, and who is now running for the Senate, has all of these qualities. In her new book - a compilation of her newsletters to constituents, interspersed with ''letters to the editor'' and op-ed newspaper pieces - she takes on a broad array of our toughest issues.

While her book is marred by repetitiveness (recurring anecdotes, arguments, and statistics have not been deleted) and would have benefited from the addition of new material, it has a great deal to tell us about how our political system works - and about the things that keep it from working.

Few politicians have inspired more crossover voting than has Representative Fenwick. Yet her staunch commitment to the Republican Party is based on her conviction that it holds the clearest vision of freedom. If you've never been quite certain how America's political parties differ in their underlying principles - rather than practice - this book may give you new insight.

She argues that a good Republican is just as sensitive to suffering as a good Democrat, but looks to private and voluntary sources for solutions, rather than to government. She shows how government programs have often stifled initiative and individuality, and have enfeebled and enraged those they sought to benefit.

These essays are marked by a hard-won, tough-minded optimism; by humor, warmth, and a search for solutions. They steer clear of easy answers and sloganeering, and refrain from attacking individuals.

Fenwick examines the quagmire of slush funds and special interests. She illustrates how our political system breeds conflicts of interest and corruption. And she offers concrete proposals for attaining a more genuine accountability and responsiveness on the part of government.

Her discussions of terrorism, violence, and moral values are deeply felt and are completely lacking in self-righteousness. She offers powerful insights into how our ''misapplication of the virtue of tolerance'' has led us to ''condone violence whenever an altruistic motive is claimed.''

Few political figures are less doctrinaire. Norman Cousins, in his Foreword, describes her as ''. . . liberal in . . . that she is deeply responsive to human needs; . . . conservative in that she is distrustful of power.''

In an essay entitled ''No Leaders, Please!'' Fenwick recalls Hitler's charisma and popularity (he garnered more than 80 percent of the vote). Quoting Bertrand Russell, she argues that to survive as a democracy, we must develop an ''immunity to eloquence.''

Fenwick's passionate love for justice and freedom makes this book compelling reading. While she makes no pretense of objectivity, the reasoning behind her positions is clearly and honestly stated. But to be as sure of your own stance as she is of hers, you'll have to do a great deal more than consider what she has to say.

A major contribution of her book is that it shows how much hard work, study, and investigation it takes to formulate a responsible viewpoint. The larger contribution is that it inspires the reader to make this effort.