While a weak economy has the national GOP uncertain of its prospects in Tuesday's elections, Florida Republicans can bank on at least one powerhouse: Marion T. Keith of Pinellas County.

Twice-widowed and hard of hearing, Keith, 83, commands the most potent Republican organization in Florida's strongest Republican county.

Its home is 60 condominium buildings at the sprawling On Top of the World complex just north of Clearwater. Top of the World is a quintessential GOP stronghold, a vast retirement village stocked with conservative, elderly, white voters drawn heavily from the nation's midwestern heartland. Sixty-five percent of its 4,357 voters are Republicans.

Prodded by Keith and her small army of volunteers, they turn out at the polls in extraordinary numbers, 86 percent in the last general election. "Every Republican wins in my precincts," she boasted recently, scanning a list of vote totals from 1980. "Presidents, governors, everybody . . . some of those people lost the election, but not at Top of the World."

Keith is party coordinator for the condominiums' two voting precincts; before it was split three years ago, Top of the World reportedly comprised the largest voting precinct in the nation. She selects poll workers, organizes rallies, conducts voter registration and distributes literature.

That last function is among her most significant, for access is tightly controlled at Top of the World, as it is at hundreds of other condominium and mobile-home retirement complexes across Florida. Thus, unless candidates have an inside track with resident activists such as Keith, expensive direct mailing is their only sure way to reach those high turnout voters.

So Florida politicians regularly visit Keith's small ground-floor apartment to solicit her aid. This year, Senate candidate Van Poole and congressional hopeful Michael Bilirakis are just two who have plopped down in what one local official calls her "hot seat."

County Commissioner John Chesnut Jr. said, "If you get ready to make a political run, you see the party chairman and you see Marion Keith -- not necessarily in that order."

She has built a reputation for influence over 28 years of work for Republicans here.

A native of Wisconsin, where her father was state GOP chairman, she joined the Clearwater Women's Republican Club immediately upon moving to Pinellas in 1954. Eventually she became club president, and still serves as program chairman. At club lunches at a local Holiday Inn, guests are asked about their hearing problems so they may be seated close enough to the featured speaker.

Since 1970, Keith has lived at Top of the World, an elaborately appointed complex featuring a huge blue globe, fountains and replicas of Greek statues in the courtyard.

"This is my baby," she said proudly. "There was nothing here politically when I came here. There was just 12 buildings, and I organized this."

She has a representative in every building and they can turn out a crowd quickly at her behest. Two years ago Rep. C.W. Bill Young (R-Fla.) wanted to assemble a large crowd for ex-president Gerald R. Ford on extremely short notice.

"The first thing I decided was, let me get ahold of Marion Keith," Young said. "Sixteen hundred people greeted Ford at Top of the World."

Her support takes on added importance in Republican primaries. She consults a few lieutenants before deciding on her favorites. Once she decides, it's hardly a guarded secret.

"I never tell anyone how to vote," she said. "They ask me and I say, 'Well, I'll tell you how I'm voting.' And I have about 100 people call me every election."

In the Sept. 7 Republican primary, she backed Senate candidate Van Poole, and at Top of the World he ran 30 percent ahead of his showing in Pinellas County and statewide.

Her secret? "It's hard work and a lot of talking. No matter where I am, a cocktail party, bridge party, you name it, I talk politics."

Soon there will be more work. Top of the World developer Sidney Colen plans to construct 30 more buildings, she says, "So that will be another precinct under my jurisdiction."