Montgomery County voters ousted the most liberal Republican in the House of Representative last November when they chose Democrat Michael D. Barnes over incumbent Newton I. Steers, according to a study of 1978 votes by Americans for Democratic Action.

The ADA, a liberal lobbying group, said that Steers voted on the liberal side 90 percent of the time during his last year in Congress a record topped only by 13 Northern Democrats.

During last year's campaign, Barnes contended that Steers did not fit the progressive mold of his Republican predecessors - Charles McC. Mathias and Gilbert Gude - and was "too conservative" for the traditionally liberal voters of Montgomery County.

The ADA analysis, however, showed that Steers' 90 percent rating tied him with Rep. Parren J. Mitchell, the Baltimore Democrat who was head of the Congressional Black Caucus last year. And it placed him ahead of liberal Democrats Barbara Mikulski, of Baltimore, who rated an 85, and Northern Virginia's Herbert E. Harris II and Joseph L. Fisher.

Harris was rated at 70 percent by the ADA; Fisher at 60.

A review of Barnes' performance in the early months of the 96th Congress indicates that voters in Maryland's 8th District got another liberal.

Barnes is compiling a voting pattern very similar to that of the Washington area's three other liberal Democratic House members - Gladys N. Spellman from the adjoining 5th District of Maryland (who got a 70 from ADA), and Harris and Fisher.

Barnes said last night that it would be "pure conjecture to say how his voting record might differ from that of his predecessor, except to acknowledge the obvious: On strict party-line issues, Barnes votes with the majority party.

The freshman Democrat has supported President Carter on 32 of 37 key votes, or 86.5 percent of the time. But Barnes is anxious to point out that he has differed with the administration on such a critical topic as decontrolling oil. Although it has not come up on the House floor, Barnes noted that three times in the Democratic caucus he voted against Carter's position, an in favor of continuing controls.

Asked if he believed the president will be an asset or a liability when he seeks re-election next year, Barnes said: "It will be difficult to make a case either way. Some will think that I have voted with him too often, and others will say I disagreed with him too much."

Among the seven House members whose districts are within or touch the Washington metropolitan area, Arlington's Fisher gave Carter the most consistent support, opposing him only four times in 35 votes.

Barnes was next in support of Carter, followed by Spellman, with 28 of 34 pro-Carter and Harris 30 of 37 pro administration votes. Another freshman, Democrat Beverly B. Byron of Frederick, was down the middle on Carter, voting with him 18 times and against him 18 times.

The area's two stauch conservaties, Maryland Republicans Robert E. Bauman and Marjorie S. Holt, gave the administration little support. Bauman voted with the president six of 37 times; Holt seven of 35 times.

Barnes, who missed only two of the 163 roll call votes recorded through the Memorial Day recess, voted most often like Spellman.

The two Maryland Democrats voted the same way on 106 roll calls, and differed on only nine. Barnes and Fisher voted on the same side 97 times, taking opposite sides on 11 votes. Barnes and Harris voted the same way on 104 roll calls and differently on 15. CAPTION: Picture 1, NEWTON I. STEERS; Picture 2, MICHAEL D. BARNES, Barnes' record shows Maryland's 8th District has elected another liberal.