Popular congressmen wage a fierce battle for Stennis seat. MISSISSIPPI SENATE RACE

The open United States Senate seat in Mississippi is the political chance of a lifetime. Just one Senate slot has turned over here since 1948, when retiring Sen. John C. Stennis succeeded the raving white supremacist from the hills, Theodore Bilbo.

One of about nine close races this year, the contest can do more than nudge the balance of power in the coming Senate. If tradition holds, the winner will be in office for decades and rise to considerable seniority.

The contenders are two popular congressmen - Republican Trent Lott and Democrat Wayne Dowdy. The race is close, and it is peculiarly Mississippian.

On the stump, the candidates are Southern classics.

Mr. Lott, the Republican from growing coastal Mississippi, is brassy and gregarious, backslapping his way through Kiwanis luncheons and chamber of commerce speeches, telling old jokes well. Mr. Dowdy, lean and slightly rumpled, is a mild, earnest, deep-country sort (and a wealthy radio station owner) - Abe Lincoln played by Jimmy Stewart.

In Congress, Lott is a major GOP leader and a true-blue Reagan conservative. Dowdy is no boll-weevil Democrat. In the newer mold of Deep South Democrats, he depends heavily on the votes of blacks, and votes accordingly - often with national Democratic leaders.

But voters are not going to hear much of national politics since the campaign is turning almost entirely on the politics of the pork barrel - which candidate can bring in the most federal dollars.

Mississippians, strong Reagan voters, run conservative on social, cultural, and defense questions. But they also run dead last among states in many measures of wealth and material progress. Federal spending is not viewed with much hostility in a state that - in the Delta region - receives $1.36 from Washington for every dollar it pays.

In spite of his conservative credentials, Lott campaigns on television as what one Democrat here calls a ``New Deal Republican.'' His ads tout him saving social security and college student loans from budget cutters, bringing highway funds to Mississippi. Dowdy is busy rebutting these claims, noting that Lott has voted for cuts in both social-security benefits and student loans.

Both claims are true - respecting different votes on different aspects of these programs.

The imagemakers in both campaigns have shown superb dagger work over the airwaves. Each strives for the low ground of down-home regular fellow.

In commercials, Lott appears in flannel shirt, driving a pickup truck, and talking highway money, heavy construction going on behind him. Dowdy appears in warm, black and white still shots, bathed in the orchestral strains of ``Appalachian Spring,'' intoning his earnest vow: ``I will never forget who I am, where I come from, or who sent me.''

But the big issue in this campaign has been George Awkward, Lott's security guard and driver.

The Dowdy team produced a spot that wove homespun scenes of country Mississippi with jarring cuts to a stretch limo barreling down a dirt road in a cloud of dust, the passenger hidden behind his Wall Street Journal. The narrator decried Lott's ``$50,000-a-year chauffeur'' and his social security cuts as the limo sped past a woman waiting at a mailbox, presumably for her social-security check.

The spot was a hit, but the Lott team hit back. Mr. Awkward, a black Capitol police officer wearing a shoulder holster and looking every inch the G-man, stares at the camera and vents his outrage at being called a chauffeur by Dowdy - an offense ``to every law-enforcement officer that puts his life on the line every day.''

On a recent Sunday, all five political columns in the Jackson Clarion-Ledger dealt with the chauffeur controversy, all five pleading for higher campaign ground.

So far, Lott is running from 8 to 9 points ahead in the polls. Most expect the race to tighten. Although Dowdy campaigned heavily last winter and early spring to win a tough primary, he was off the airwaves till early last month. Lott has been advertising through the summer.

The Lott camp is taking the battle to all fronts, even competing for votes in Dowdy's own congressional district. Lott's grass-roots organization is reportedly one of the strongest ever for a Republican in the state.

Dowdy says the race will be won or lost among the Reagan Democrats of northern Mississippi. Many are older and have long memories of federal programs improving their lives, from the Tennessee Valley Authority to medicare, says Dowdy spokesman Dean Pittman.