You might think that after 16 years in the Virginia Senate and a senior leadership role in its majority party, Democratic Sen. Joseph V. Gartlan Jr. would take reelection for granted.

But if you thought that, you wouldn't understand Fairfax County's 36th District or a determined Republican named C. Ronald (Ron) Smith.

Smith and Gartlan agree on at least one thing about the area they are fighting to represent: No politician, either Democrat or Republican, gets a free ride there.

The 36th District encompasses southeast Fairfax County, from Alexandria on the north to I-95 on the west, and its partisan loyalties are cleanly split.

Smith estimates, and Gartlan acknowledges, that in any election about 40 percent of the district's voters will vote Republican, 35 percent will vote Democratic and the remaining 25 percent will determine the outcome. The district has supported Republicans such as President Reagan and 8th District Rep. Stan Parris, and Democrats such as former governor Charles S. Robb and Gov. Gerald L. Baliles.

Add to this volatile atmosphere the high voter turnover that is common in Fairfax County, and even an entrenched incumbent can't relax in the 36th.

"The Republicans can run Bonzo and get 40 percent," Gartlan pithily observed. "Give Bonzo a campaign manager and that can mean trouble."

Smith readily admits that he is the underdog in the 36th, but no one, including Gartlan, compares Smith to Bonzo. A lawyer and former Capitol Hill staffer, Smith is betting that political skills that worked in Washington will do just as well at the state Capitol in Richmond.

"I think the opportunity is ripe for a real upset," Smith said. "Look at the numbers in the district. And I truly believe that I am more in tune with the voters philosophically than he is. I've got the freshness, enthusiasm and the edge that someone new has. I believe we're going to do it."

Gartlan thinks otherwise. "I don't mind defending my record, but my opponent is making his most slashing attack on my alleged ineffectiveness," he said. "If you look at what we've done for Northern Virginia, I don't know where he comes off."

Gartlan, a 62-year-old retired lawyer, enjoys the advantages that flow to a senior member of the party that dominates the General Assembly. He is chairman of the Senate's Privileges and Elections Committee, which has jurisdiction over all of the state's elections. He was rated the fifth most effective member of the Senate in a newspaper poll earlier this year.

And he has shown a dogged determination in fights on the Senate floor. Earlier this year, Gartlan led the effort to censure Sen. Peter K. Babalas, a Norfolk Democrat, for supporting legislation that helped one of his legal clients. Gartlan says that publicly taking to task a senior member of his own party was "very, very difficult."

On the other hand, Gartlan contends that Smith has no similar record of accomplishments. He points to Smith's relative lack of involvement in local civic organizations and his failure to register to vote in the district until last year.

Smith, 39, a longtime aide to former Texas Sen. John Tower, says he maintained his voter registration in Texas until last year out of loyalty to Tower. And he says his political work has made it impossible to take on a long list of other duties.

"Every job I've ever had I work at 18 hours a day," Smith said. "I don't have hobbies or a lot of free time. I pour myself into work, and I would pour myself into this job."

Smith contends that Gartlan, despite his seniority, has failed to concentrate on the issues that matter most to his district.

"Any fool knows that by far the number one concern here is traffic," Smith said. "But it's obvious that he has not been a leader on this issue, and has been a follower at best."

Gartlan, Smith said, "has not done a whole lot other than raise taxes" to solve the area's growing traffic problems. Gartlan voted last year for a package of tax increases that will pay for additional highway construction; Smith said he would have opposed the one-half cent increase in the sales tax.

Smith also contends that he differs with Gartlan on several other issues. Gartlan opposes the death penalty for criminals, while Smith supports it. Gartlan voted to hold a referendum on a proposed state lottery, while Smith says he would have opposed it.

Smith says that he opposes collective bargaining by public employes and that Gartlan has supported legislation that would make it easier for state workers, including teachers, to negotiate with their employer.

Gartlan has a big advantage over Smith in one important area: campaign money. Gartlan says he has raised nearly $60,000 and has had no trouble picking up contributions. Smith says he has spent about $35,000 -- about $25,000 of it in the GOP primary -- and is about $15,000 in debt.

"I don't think my opponent can win this election, but I think we could lose it," Gartlan said. "I just have to go out and show people what my record really is."

Smith said that if he can get his message out to enough voters, he can win.

"I've been working at government and politics at the national level since I was in high school," he said. "And I know I can do the job at the state level as well."