It's seldom that Rep. Gladys N. Spellman (D-Md.) deserts the District on a key vote in the House, and rarer yet when she agrees with two conservative colleagues in the Maryland House delegation, Republicans Marjorie S. Holt and Robert E. Bauman.

But Spellman did both when she voted against funds for the proposed D.C. convention center.

"Far too costly, akin to a Taj Mahal," the liberal Spellman said in explaining her opposition to the $110 million item in the city budget - which survived by a vote of 199-190 last week with little help from the normally pro-District Maryland delegation.

Spellman acknowledged that her fiscal concern was matched by the lingering hope that if the city plan fails, a convention center may rise someday among the garden apartments and shopping centers in her own suburban Price George's County.

Similar pride in their hometown of Baltimore, where ground has been broken for a harbor-front convention center, resulted in the failure of two other liberal Democrats, Barbara A. Mikulski and Parren J. Mitchell, to support the proposed District convention center.

Mitchell, head of the House Black Caucus and a good friend of D.C. Del. Walter E. Fauntroy, voted "present" (neither for against), a practice known in the parlance of the Maryland legislature as "taking a walk."

"I didn't want to hurt Washington," Mitchell said, "but I have strong feelings that two convention centers are not needed in what is more and more becoming the twin cities of Washington and Baltimore."

A Mikulski aide said, "Barbara's very pro-Baltimore, pro-tourism," in explaining her vote to table an amendment that added funds for the convention center on the budget.

The only member of the Maryland delegation who voted in favor of the center was Rep. Newton I. Steers of Montgomery County, who said the center would "help the area's economy in the long run" by attracting major conventions.

Siding with Steers in favor of the center were Virginia's two suburban Democratic representatives, Herbert E. Harris II and Joseph L. Fisher. About the only issue that can coax those three - and Spellman - to line up against the District is the perennial proposal to tax suburban commuters to the city.

After permitting herself to echo the conservative line that cost overruns on a District convention center would be "a burden to the nation's taxpayers," Spellman allowed that a center "of comparable size and sufficient luxury, financed by the private sector, could be built at a much lower cost in Prince George's County."

The idea of a convention center in Prince George's first came up during the 1977 session of the Maryland General Assembly when that county's powerful delegation attempted unsuccessfully to get $20 million worth of state supported bonds committed to a convention site near the Capital Centre, home arena of Washington's professional basketball and hockey franchises.

John J. Lally, chief aide to Prince George's County Executive Winfield M. Kelly Jr., said the county's proposal was not renewed in the Maryland legislature this year "because D.C. said its pains were six minutes apart. It's coming, they said," of the downtown convention center.

Lally said the original proposal of developer William Levitt to build a convention facility near the sports arena in Largo, and several other proposals near Metro stations, can be revived if the District plan fails to materialize.

"We frankly don't believe it will ever be built downtown," Lally said. "We wish the District well, but in our bones we don't think it will happen."

The suburban convention center would be financed with $20 million in bonds, approved at either a state, regional or bicounty level, Lally said."And the private developer would guarantee the bonding authority that any cost overruns would be his," Lally added.

The motion to table funds for the city center was proposed by Bauman, the self-styled conservative watchdog of the House, who said that he "couldn't care less" about rival convention centers. His concern was about "fiscal irresponsibility" of the proposal, which he said would result in "sticking the taxpayers with a subsidy" to cover operational losses.

The issue still is not settled, however. The House-approved compromise on $1.2 billion D.C. budget still must be acted upon by the Senate.