"This may be the most important TV program you have ever watched," says the handsome congressman dressed in carefully tailored, dark pin-stripes that immediately communicate a tone of quiet conversatism.

Then comes the message delivered in the resonant, professional tones of an off-screen announcer: "THERE IS AN AMERICAN CANAL AT PANAMA."

That's the opening of a glossy, 30-minute color TV firm unvieled yesterday by the American Conservative Union as the newest weapon in its campaign to block Senate approval of President Carter's Panama Canal treaties.

Between Oct. 29 and Nov. 13, the ACU will use the film in a media blitz of Texas Louisiana and Florida, airing it on 29 television stations in the three states.

Rep. Philip Crane (R-Ill.), the ACU chairman, said the move has a double-barrelled purpose to spur viewers into calling on their senators to vote against the treaties and to contribute funds to the ACU's anti-treaty war chest.

"We're startling in those three states because they are places where sentiment against the treaties is very strong," Crane said. "Our hope is to generate enough contributions to get this films arred in all of the 50 states."

Although the ACU had raised approximately $400,000 for its fight against the treaties. Crane said, most of the money has been spent first on newspaper ads urging voters to put pressure on their senators and then on producing the TV documentary.

Crane said the film, made by a Miami advertising agency, cost $20,000 to produce, putting it on the air in the three states, he added, involves additional costs of $20,000 to buy time and $6,000 to advertise it in local newspapers.

In the film Crane sets as the "host," telling viewer at the outset: "The President and most senators are determined to give away the American canal at the isthmus of Panama - unless you demand otherwise. This is your chance to be heard - and in the next several moments, I'll tell you how."

He then introduces a supporting cast of outspoken treaty opponents that includes Sens. Strom Thurmond (R.-S.C.), Jesse Helms (R.-N.C.), Paul Laxalt (R-Nev.) and Jake Garn (R-Utah).

Also featured in the program are Jaj. Gen. J. Milnor Roberts Jr., executive director of the Reserve Officers Assocation: Phelps Jones of the Veterans of Foreign Wars and Guthrie F. Crowe, a retired U.S. district court judge for the Canal Zone.

Each takes a turn before the cameras to argue that Carter must be denied the 67 votes - two-thirds of the Senate - necessary to approve the treaties turning gradual control of the canal over to Panama.

Between them, they manage to pack into the film's 30 minutes the entire catalogue of complaints that have been made against the treaties. But two, in particular, are given special emphasis:

That the canal is an important national asset, built, paid for and legally owned by the United States.

That U.S. security would be endangered and U.S! prestige would be disminished by surrending to "blackmail" from an unstable and leftist Panamanian dictatorship.

In between the repetitions of this message. Crane gives viewers detailed instructions, right down to the zip code, on how to write their senators and to argue that they call a toll-free number to "pledge a contribution of $10 or more so we can continue our TV campaign against the canal giveaway.

At yesterday's preview of the film, several reporters noted that the most potentially effective opponent of the treaties in American politics - former California Gov. Ronald Reagan - was conspicuously absent from the cast.

Crane replied that Reagan's schedule had not permitted him to come to Washington where the show was taped two weeks ago. The ACU, he said, felt it was more important to get the film made and on the air as soon as possible than to wait for a time when Reagan would be available.