Two bitter political rivals--Democratic Del. Earl E. Bell and Republican Leesburg Mayor Kenneth B. Rollins -- are battling for Loudoun County's seat in the Virginia General Assembly in a campaign that will test GOP strength in Washington's outer suburbs.

The state Republican Party has targeted the race for extra publicity and financial aid in an attempt to capture the single-member district created in this year's controversial House of Delegates redistricting plan. GOP strategists believe the plan gives them an excellent chance of ousting Bell, a three-term incumbent, who has been serving as one of three delegates from Prince William and Loudoun counties.

Rollins, a former legislator who is said to be leading in two polls, terms Bell "totally ineffective." Bell, in turn, has accused Rollins of lying. Amid the pointed rhetoric, the weekly Loudoun Times-Mirror urged both candidates to end their "name-calling" in an editorial complaining of "bitterness, old resentments and pettiness."

Bell has been furious over Rollins advertising, especially one ad that says Bell "failed to get a single . . . bill passed in the House this term." Waving a legislative computer printout, Bell said he was the chief sponsor of five bills enacted this year. "You've heard of the big lie, haven't you? That's what he's using, the big lie," Bell says.

Rollins responds that his "intent" in the ad was to refer to the 1980, rather than 1981, General Assembly session. "Maybe the language there was wrong and we'll certainly correct it," he adds.

Bell and two other Democrats ousted Rollins, then an independent, from the House in 1975 and outpolled him again in 1979. "I beat him twice. Unfortunately he keeps coming back," Bell complains. "I would personally like to bury him this time."

In the 1979 contest, however, Rollins drew more votes than Bell in Loudoun, although his margin there was too slim to overcome Bell's lead in Prince William.

"I better work like hell for the next two weeks," says Bell, 61, predicting a "tight race." Rollins, 45, a lawyer, part-time mayor and a substitute state judge, asserts: "We're substantially ahead."

Virginia Republicans scored dramatic gains in largely Democratic suburbs during the 1970s, winning legislative victories in Fairfax County and Alexandria and seizing control of the Arlington County Board. In Loudoun, nevertheless, Democrats have remained dominant, though several recent elections hinted at rising GOP strength. Republican Sheriff Donald Lacy, now under investigation for alleged financial and sexual improprieties, upset a Democratic incumbent two years ago. Last year, Republican Rep. Frank R. Wolf carried Loudoun by a 6-to-4 margin as he defeated incumbent Democratic Rep. Joseph L. Fisher for the 10th District congressional seat.

These gains help explain the state GOP's interest in the Loudoun race. Neil Cotiaux, spokesman for the Virginia Republican Party, says Rollins is among a number of House candidates getting special backing in Northern Virginia, the Richmond area, Virginia Beach and elsewhere. Rollins says he has received $500 from the state party so far, and he was among several Virginia Republicans invited to the White House for a publicized talk with President Reagan a few weeks ago.

The winner in the Nov. 3 election faces the prospect of another contest next year. The redistricting plan that established the single-member Loudoun district was ruled unconstitutional by a three-judge federal panel in Richmond, whose members objected to wide disparities in voting populations. Nevertheless, the panel allowed this year's elections to take place.

In separate polls this month, the Times-Mirror and WAGE, a Loudoun radio station, reported identical results giving Rollins a lead of 13 percentage points. The polls showed 41 percent for Rollins, 28 percent for Bell and 31 percent undecided.

Rollins has sought to make highways a central issue, asserting that Bell, a Leesburg automobile dealer, has proven ineffective in seeking improvements in Loudoun's congested thoroughfares. "I'd be very happy to make this election a referendum on roads in this county," Rollins said during a campaign debate recently.

Rollins argues that Loudoun should "jawbone" with state highway officials to gain quicker action for county roads, and he wants legislation to shift more highway money to suburban counties. Bell attributes highway problems to a statewide shortage of construction funds and he expresses doubt that the General Assembly will change its formula for distributing highway money in the near future.

Whether the highly publicized investigation of Sheriff Lacy will affect the Rollins-Bell race is unclear. Lacy resigned as comanager of Rollins' campaign. Both candidates say they believe the Lacy probe has not been an issue in their contest.

Rollins says he expects to spend $12,000 to $15,000 on the campaign. Bell says he plans to spend $10,000 to $12,000. Neither candidate has yet filed a financing report.