RICHMOND, JAN. 22 -- A proposal to build an outer beltway to divert traffic around the Washington suburbs, which has long divided environmentalists, developers and other groups, is now dividing Northern Virginia's General Assembly delegation.

Sen. Charles L. Waddell (D-Loudoun), chairman of the Senate Transportation Committee, introduced a resolution today for the legislature to endorse a western bypass that would cut through Loudoun County, Waddell said, preferably near Dulles International Airport.

In a highly unusual move, Sen. Joseph V. Gartlan Jr. (D-Mount Vernon), sent a memo to every legislator urging them not to support the resolution, saying the proposed bypass "will create environmentally damaging, and expensive, sprawling development . . . . I believe it inappropriate to express support for a project which will cost the commonwealth an inordinate sum of money when there remain serious questions as to the benefits."

Advocates say a bypass is needed to relieve congestion on the Capital Beltway by diverting north-south interstate traffic in a broad, looping arc around the Washington region. Both the eastern and western proposals would cost about $1.4 billion to $1.7 billion in today's dollars. Critics say the western bypass would not do as much to improve traffic.

Many Northern Viginia politicians favor an eastern alignment for any Washington bypass, which would skirt Northern Virginia altogether, arguing that it would provide more relief for traffic congestion on the Capital Beltway. In addition, slow-growth advocates believe a western bypass would simply become another development corridor that would further clog roads and harm the environment.

Waddell's resolution would order the Commonwealth Transportation Board to oppose any eastern bypass route unless Maryland has first or concurrently approved a western route. It calls for the road to be paid for by the federal government or with tolls or both.

Waddell came under fire from anti-development groups and local government officials last year for sponsoring legislation backed by developers that overturned Fairfax County's development limits in the Route 28 corridor.

Slow-growth activists also have criticized Waddell because about a third of his 1987 campaign contributions came from developers and builders.

In this session, a member of Waddell's staff, former Fairfax Chamber of Commerce president and bank official Linda Wright is the volunteer head of the Northern Virginia Transportation Alliance, a developer-funded transportation lobbying group that supports the western bypass.

The bypass resolution has also set Waddell in opposition to some of the region's governments, all of which fought his Route 28 bill last year. The Fairfax and Prince William boards of supervisors favor an eastern bypass route, with Fairfax opposed to any allignment near Dulles and Prince William opposed to any route that crosses its border. The Loudoun board is split on the issue, with a slim majority recently flip-flopping to favor a route near Dulles.

In a letter to the region's delegation, Fairfax Board Chairman Audrey Moore urged that the bill be killed, saying an outer beltway could divert as much as $2 billion from more important projects, such as improvements to Interstates 66 and 495.

The Northern Vigrinia delegation, which was deeply divided over Waddell's Route 28 bill, appears to be split on the bypass proposal as well.

Sen. Clive L. DuVal 2d (D-McLean), head of the delegation, said he supports an eastern route for the bypass and therefore would not support Waddell's resolution.