Henry Fonda looks as iconographically American now as he did 40 years ago when he was telling Ma Joad, in "The Grapes of Wrath," that a man's soul is jest a piece o' one big soul. Boy! Fonda's presence is what really elevates "Gideon's Trumpet," a Hallmark Hall of Fame CBS movie at 9 on Channel 9, into the realm of the impressive and the important.

Unfortunately, the script by David W. Rintels tends to be more painstaking and methodical than emotionally involving. It is based on the 1964 Anthony Lewis book about the poor Florida man whose conviction on petty larceny charges was eventually overturned and became a landmark in legal history and civil liberties.

Sentenced to the maximum five years -- because he had prior convictions on similar charges -- Clarence Earl Gideon set out to study law in the prison library and became convinced he was denied due process at his trial because he was too poor to afford a lawyer and the court refused to appoint one for him.

Because of that case, it is now the law of the land that every accused person is entitled to legal counsel. But legal arguments and courtroom debates are not easily dramatized, and Rintels has problems keeping these scenes from becoming dry and static.

Fonda, however, is so magnetic and resonant a figure now -- a national monument, as his son, Peter has called him -- that he compensates for a certain laxness in the script and the overly austere direction of Robert Collins. The most Fonda-esque moment comes when Gideon gets to announce the Court's decision to his fellow prison inmates: "Nine to nothin' -- yew-nanimous!"

There is some residual impact merely from seeing the 75-year-old Fonda look old, and not made up to look younger; it was an asset as well in NBC's recently televised broadcast of the Preston Jones play "The Oldest Living Graduate" with Fonda in the title role. That he still leans toward roles with social significance, as in "Gideon," gives the old-timer a combative dignity it would be foolish to begrudge him.

Fonda is joined by such other ultra-venerables as Fay Wray as Gideon's loyal landlady, John Houseman as the Chief Justice, and Sam Jaffe as another justice. Journalist Lewis himself has a cameo role as a reporter, and Jose Ferrer does a very capable job as attorney Abe Fortas, later a Supreme Court justice himself.

Considering the general surplus of bad news now clogging the air, and considering the poor press the Supreme Court's gotten of late, the film does have a certain affirmative kick. The dryness could be interpreted as a documentary approach; about the only fanfare-for-the-common-man music is used when Gideon marches ceremoniously to the mailbox to send off his letter.

In other words, one couldn't say the producers had pulled out all the stops and made a manipulative "Norma Rae" of jurisprudence, but the film would probably be more satisfying if they had.