When Ann Riley Smith travels south through state Senate District 28, she undergoes a remarkable transformation.

The Democratic challenger starts her journey as a plausible political contender and ends it as an almost impossible long shot in her bid to unseat Republican incumbent John H. Chichester.

The 28th District is as geographically enormous as it is politically diverse.

At its northern end, it includes the suburbs of southern Prince William County, home to thousands of commuting professionals who might respond favorably in the Nov. 3 election to a Democrat of Riley Smith's background and views.

But the 28th stretches and stretches and stretches -- more than 120 miles south, through Stafford County and Fredericksburg, all the way to Lancaster County on Virginia's Northern Neck. Most voters there aren't likely to turn away from Chichester, according to political observers from both parties.

This leaves Chichester, a 50-year-old conservative from a well-known political family who made an unsuccessful bid for lieutenant governor two years ago, the heavy favorite to return to the Senate for his third full term -- at least according to devotees of the conventional political wisdom.

Riley Smith is no devotee. The 48-year-old Democrat, a member of the Stafford County Planning Commission, acknowledges that she is an underdog, but said her campaign, which stresses attacks on the incumbent for what Riley Smith describes as his aloof political style, could win big in Prince William and north Stafford counties, and do well enough elsewhere to produce an upset.

"He's got a race on his hands," Riley Smith said, charging that her opponent repeatedly has refused to listen to his constituents. "He's been short and curt, almost to the point of being rude."

If Riley Smith's allegations about Chichester's abrasiveness are true -- he denies them vigorously -- it is also true that the incumbent can be a model of Virginia gentility when he wishes. During an interview last week in front of the Stafford courthouse, Chichester was in an affable and talkative mood, and radiated the kind of confidence that allows him to claim that he has almost no knowledge of his opponent's views.

Chichester said his advocacy of better transportation and education, balanced by his conservatism on fiscal matters, will again prove popular across the 28th District, even in Democratic-leaning Prince William.

"I've kept an eye out for the taxpayers," he said. "The one-party system doesn't work. It doesn't work in Poland, it doesn't work in Cuba and it doesn't work in Virginia."

One of the main issues in the campaign is the question of how closely a General Assembly member should hew to the wishes of local governing bodies. Specifically, Riley Smith has scored Chichester for his opposition to the Stafford County Board of Supervisors' efforts to build an airport along the I-95 corridor.

Chichester described the proposed airport as a monumental waste of taxpayers' money and scoffed at the notion that as a state senator he should be expected to uncritically support any idea advanced by a county board. Riley Smith "thinks I ought to be a rubber stamp for local government," said Chichester. "No 'yes man' am I."

Chichester, who owns a Fredericksburg insurance agency, was first elected to the Senate in a special election in 1978 to fill an unfinished term. His father served as Stafford County commonwealth's attorney, a position his brother now holds.

Chichester said he was embittered by his loss two years ago to Democrat L. Douglas Wilder in the lieutenant governor's race, expressing particular disdain at the media for what he described as unfair coverage of allegations that he used his position in the legislature to help the insurance industry. Contrary to some reports, Chichester said, he remains enthusiastic about his duties in the Senate.

Riley Smith, who owns a private detective and bail bond firm, grew up in Prince William County and has lived in the Stafford community of Aquia Harbor for several years.

Chichester said he plans to spend about $30,000 in the race; Riley Smith said she plans to spend about $28,000 and is concentrating on cable television advertising in Prince William and Stafford and radio advertising throughout the district.