It would be hard to imagine a more dismal doorstep to the nation's capital than the Portal site, the 10-acre tract at the foot of the 14th Street bridge that serves as a monument to a lack of foresight.

In stark contrast to the elegance of the nearby Jefferson Memorial, Portal greets visitors to Washington with a collection of weeds, a parking lot filled with hundreds of cars of government workers, the noise of Conrail trains rumbling into the city and the dust of a U.S. General Services Administration facility that rattles coal to shake it apart.

On Thursday, however, the District's urban renewal agency plans once again to attempt to start transforming the site, the last major redevelopment parcel in Southwest Washington, by picking one of three development teams seeking to turn the location into a glittery, multimillion-dollar complex of offices, shops and a hotel.

"It's probably the most critical decision we'll make this year," said Redevelopment Land Agency board member Kwasi Holman, who heads the city's business and economic development office. "It's probably the key remaining urban renewal site we have."

Robert Mendelsohn, one on the long list of developers seeking to win the right to build on the site, said, "At present, it's a singularly unattractive, almost invisible entrance to the city. There's a void there. There is no there there."

But James E. Clay, chairman of the five-member redevelopment agency, can envision how Portal, bounded by 12th, 14th and D streets and Maine Avenue SW, might one day be revitalized from its forlorn state.

"It's the gateway to the city, and a very important one," he said, "because of the number of people who come into the city through National Airport and then drive across the 14th Street bridge . Portal is important to the city from an aesthetic and economic standpoint."

Some of the biggest names in the nation and the Washington area are vying for the prize: probably, the right to pay the city more than$50 million for the land and then spend upwards of $250 million to build the project. Together, the three teams already have spent more than $1 million in architectural, engineering and legal fees to prepare their development proposals.

One entry, called Portal Associates, includes Washington developer Conrad Cafritz, Boston builder Mortimer B. Zuckerman, who recently bought U.S. News & World Report, and Roger R. Blunt Sr., who heads one of the largest black-owned construction companies in the nation and is the next president of the Metropolitan Washington Board of Trade.

Another development team, Portals Development Associates, is headed by Western Development Corp., developers of the Georgetown Park shopping mall and the controversial Washington Harbour condominium, shopping and office complex nearing completion along the Potomac River shoreline in Georgetown.

The third group, Rock-Portal Inc., includes Rodman C. Rockefeller, son of the late Nelson A. Rockefeller, Marathon U.S. Realties Inc., a subsidiary of the giant Canadian Pacific rail line, and local developers Farr-Jewett Portal Partners and First City Associates. They have hired James W. Rouse, the development guru who shepherded the new town of Columbia, Md., into existence and designed the popular Harborplace eating and shopping malls on the Baltimore waterfront, to devise a shopping arcade along the Southwest waterfront that would connect with the Portal site.

All the teams have substantial minority ownership, with Blunt having the single biggest role in any of the proposed ventures with 28 1/3 percent ownership. Three other minority-owned firms each have 1 2/3 percent ownership in Portal Associates.

Portals Development, the Western-led group, is 49 percent minority controlled, with Eastcoast Development Corp. controlling 8 percent and seven Washington lawyers, consultants and others 5 percent to 8 percent apiece.

Rock-Portal is 25 percent minority-controlled, with lawyers Keith Seay and Chester Davenport controlling 22 percent through their First City Associates development group, while Howard University holds a 3 percent equity share that it bought for $1.

All three development groups have pledged to contribute $100,000 or more annually to various Southwest Washington community, tenant and cultural groups.

"We have three quality proposals," Clay said. "The board, whichever way it decides to go, will be able to wrap up a deal quickly and sign a contract."

Three years ago, the urban renewal board awarded development rights to Banneker Associates, a team that included shopping center magnate Theodore N. Lerner and developer Melvin Lenkin. But by October 1983 the agency rescinded its approval of Banneker, when the Redevelopment Land Agency and the developers could not agree on a price for the land. The developers originally offered $45 million for the site, but later reduced the figure to $38 million.

The redevelopment board reopened the bidding this year, but limited it to the five groups that made proposals last time. Banneker dropped out, and so did a group headed by parking lot magnate Dominic F. Antonelli Jr., leaving three revamped groups.

The redevelopment agency's latest appraisal lists the site as worth $51.6 million, although Clay said that "everything is negotiable."

Having been burned by its first-choice selection last time, the agency hopes to avert that situation this time by ranking all three developers so that if negotiations founder with the winner, the agency could immediately turn to the second choice.

The redevelopment board is faced with sifting through three widely different designs for the site.

The proposal of Portal Associates, the Cafritz-Zuckerman-Blunt team, is the most architecturally traditional, Rock-Portal's is the most ambitious, and Portals Development's is somewhere in between.

Portal Associates' $250 million plan, designed by the Washington office of the Skidmore, Owings and Merrill architectural firm, calls for nine traditional-looking office buildings, "which are simple and quiet," in the words of architect David Childs. The plan calls for a 370-room Holiday Inn, a little retail space, and leaving open a portion of the Conrail tracks that cross diagonally through the site.

The $460 million Portals plan, drafted by Washington architect Arthur Cotton Moore, and the$368 million Rock-Portal design, by the New York firm of Brennan Beer Gorman, call for decking over most of the railroad tracks and more space for retail stores.

Moore's design, in some respects similar to his Washington Harbour project on the Georgetown waterfront, includes arches, turrets and curving structures, a 600-room Radisson Hotel, pools of water on the plaza over the railroad, and a walkway over the Southwest Freeway to a small, passive park along the Southwest waterfront.

The would-be park is not part of the Portal site, but the land is owned by the city government and adjoins the Washington Marina. Its proposed use has led to a verbal sparring match between principals of two of the development teams.

The Rock-Portal plan, which includes a 300-room Marriott Hotel, calls for construction of a Rouse-designed bridge over the freeway and a shopping arcade along the waterfront.

The Environmental Policy Institute, a national conservation organization, has voiced support for passive use of the land, saying that the Rock-Portal plan "could create congestion at this bottleneck point."

Mendelsohn, the project coordinator for the Portals Development proposal, said, "We think it would be absolutely inappropriate to build some large commercial development" on the waterfront land across the freeway from the Portal site.

Rockefeller said that "Americans have allowed their waterfronts to be captured first by railroads, then manufacturing, then highways. You don't build a passive park. Put commercial activity in there and you make it useful for up to 18 hours a day."

Rouse said: "I just can't believe that people on the waterfront is a misuse. It doesn't add so much in commercial space as in life."