FAVORITE SON By Steve Sohmer Bantam. 484 pp. $18.95

IF YOU GET a lump in your throat when you see Mount Rushmore and marvel at what the Founding Fathers wrought at Philadelphia, steer clear of this book and curl up with a copy of The Federalist. Steve Sohmer's cast of characters may be listening to the heartbeat of America, but only after they've dosed it with strychnine. The result is an engaging and entertaining Washington political thriller liberally laced with sleaze, treachery, conspiracy and blackmail. And, for the most part, Favorite Son really delivers.

Sohmer, a former executive at Columbia Pictures and at NBC and CBS Entertainment, skillfully moves the action around in pedal-to-the-floor fashion from the marble corridors of power in Washington to the jungles of Honduras and a nunnery in Cleveland. Everybody plays hardball here. Consider the CIA director and his co-conspirator, the White House hatchet man, as they preside over the opening of a new frontier in viral warfare and a plan to assassinate a popular Nicaraguan contra leader, Colonel Octavio Martinez: "They had a poison that couldn't be traced and a method of innoculation that couldn't be detected. In six months or a year, Martinez would develop sarcomas and be diagnosed with AIDS. His friends and brothers-in-arms would shun him. His wife would desert him. The Church would castigate him. And he'd stumble off to die in obscurity . . ." Rasputin and Machiavelli would be lucky to be replacement players on this team.

Sohmer dispenses with literary, let's-meet-the-characters niceties and gets right down to it: Favorite Son opens with Martinez, and dashing young contraphile Senator Terry Fallon, sauntering out onto the Capitol steps for a welcoming ceremony. A drop-dead accurate sniper promptly ices Martinez and wounds Fallon as well.

This unforeseen development is met with dismay at the CIA -- not only had the Company had already marked the late Colonel Martinez for a more ingenious demise, but the assassin turns out to be a rogue Agency alumnus gone freelance. The hit gets mixed reviews at 1600 Pennsylvania Ave., where the nearly martyred Fallon comes to be seen as the ideal running mate for the president's flagging renomination effort. In the office of the incumbent vice-president it is seen as an unmitigated disaster; aides scramble to keep their boss from slipping from renominee to jettisonee in favor of the squeaky-clean Fallon. When their prelimnary background check on Fallon comes up pure Eagle Scout, one prescient staffer concludes: "With this guy, it's going to be something real dark and real secret and real, real ugly." And it is.

Sohmer is a better storyteller than novelist. The FBI agents assigned to the bureau's perfunctory investigation of the case -- neophyte Ross and hardened short-timer Mancuso ("I got three months till fifty-five-and-out.") -- are credibly crafted. Some of the dialogue is right on the money: "Lou, it's just like politics," the CIA director tells a White House aide concerned about the dark side of the Agency's craft. "Only the losers get to die." But Sohmers also has characters spout such hackneyed gems as "Forgive me. I always knew justice was blind. I didn't know she was stupid," and his prose contains the likes of "Yet he knew, even at first glance, that it was a face he could never forget."

Fortunately, the action is so fast-paced and graphic that you don't dwell on these occasional low points. As they probe beneath the seemingly open-and-shut assassination, the good-hearted Ross and seen-everything Mansuco ("You're talking to the guy who put the microphone under Martin Luther King's bed.") unearth a byzantine scandal that threatens to engulf not only the CIA, but the FBI, the Oval Office and even the Fallon, who goes on the offensive with the Political Damage Containment Speech of the Century. In a winner-take-all speech-off with Fallon, Nixon would have given Checkers a one-way ticket to the pound.

Sensitive readers should be alerted to the fact that some of the family ties that bind in Favorite Son really and truly do bind. And that one major player, beneath the goody two-shoes earnestness, leads a seamy, steamy secret life which leaves no doubt that in an earlier life she'd have been a natural as the Welcome Wagon Hostess in Sodom and Gomorrah.

These are, in the main, extremely bad people. And Favorite Son is, in the main, extremely good reading.

Rory Quirk, a Washington attorney, regularly reviews popular fiction for Book World.