Knoxville, Tenn. — Tennessee Democrats are fond of saying, ''If you cross the mountains no worse than 100,000 votes behind, you've got a chance.''
What they mean is that their candidates for statewide office can't always expect to do well in predominantly Republican eastern Tennessee at election time. But across the Cumberland Mountains in the middle and western sections of the state which account for 60 percent of the vote, and where Democrats hold a wide edge, the potential for making up a modest GOP lead is obvious.
This year's race for governor may be the exception that proves that rule.
Although Reaganomics is just as much on trial here as elsewhere, not even this geopolitical pattern may determine the outcome of the contest between Republican incumbent Lamar Alexander and his challenger, Knoxville Mayor Randy Tyree.
In fact, if current indications are any guide, Governor Alexander will be returned to office comfortably despite:
* A record in office that his critics call unimaginative.
* A state unemployment rate of 11.6 percent, one of the highest in the Sunbelt.
* Mr. Tyree's record of success against popular Republican incumbents. He won the mayorship of this eastern Tennessee city in 1975, when his opponent was favored, and then was reelected easily in 1979.
* The fact that in the other major statewide race here - for US senator - incumbent James Sasser (D) apparently holds a commanding lead over his widely known Republican challenger, US Rep. Robin Beard. Also likely to be defeated in her bid for a House seat from Tennessee, voter surveys show, is Republican Cissy Baker, daughter of US Senate majority leader Howard H. Baker.
But according to the latest statewide poll, released late last week by a consortium of television stations and newspapers, Alexander holds a 2-to-1 edge over Tyree. With seven weeks left until the general election and no compelling issues for voters to focus on other than the economy, seasoned political observers rate that lead extremely difficult to overtake.
''We expect (it) will close up some, but we feel pretty good about where we are right now,'' says a spokeswoman for the Alexander Committee in Nashville, the capital. Indeed, the Alexander camp doesn't feel the need of any campaign help from big-name Republicans outside of Tennessee.
Last week the candidates concluded a series of three face-to-face debates in four days, beginning in the western city of Jackson, moving to Nashville, and winding up here. Tyree hammered away relentlessly at the economy, asking voters - as candidate Ronald Reagan did before the 1980 presidential election: ''Are you better off today than you were four years ago?''
The debates undoubtedly stimulated new interest in the race and heightened Tyree's previously low statewide visibility. But Alexander is a polished public speaker, and few observers seem willing to judge that he lost much of his edge in these sessions. One marketing expert concluded that Tyree's performance had improved markedly over the course of the three debates, and the mayor says he is eager for more of them. But there apparently will be no more; Alexander has indicated he now wants to return to personal campaigning until the election.
Alexander had no opposition in the Republican primary last month, whereas Tyree overcame a better-known opponent - Anna Belle Clement O'Brien, sister of former governor Frank Clement - to win the Democratic nomination.