Newton I. Steers, Montgomery County's millionaire-turned-politician, yesterday earned the chance to reclaim the county's congressional seat for the Republican Party and avenge his 1978 defeat by Democrat Michael D. Barnes.

Final vote totals last night showed the 62-year-old Steers capturing 42.1 percent of the vote, handily defeating Constance A. Morella, a former college English professor who for 16 months has represented the county in Maryland's House of Delegates. Morella received 34.1 percent of the 37,207 votes cast.

Running well behind the two leaders in the race were State Del. Robin Ficker, whose contentious campaign style won him attention but little affection, and Philip Buford, a conservative engineer who also is a member of Montgomery's Republican Central Committee.

Ficker received 19.4 percent of the vote, with a total of 7,219 votes. Buford polled 1,602 votes, or 4.3 percent.

No one opposed the 37-year-old Barnes in the Democratic primary.

In Steers' Bethesda headquarters last night, beer was flowing and jubilant campaign workers were exchanging kisses and cheers when the candidate entered and declared, "I'm looking forward with great relish to campaigning against Mike Barnes. . . I'm going to do everything different this time."

Flanked by his wife, Gabriele, and a crowd that included a large number of teen-agers who had manned his telephone banks during the last weekend, Steers started off his general election campaign by declaring, "Barnes has showed himself to be a rubber stamp for the Democratic leadership in the House."

On hearing of Steers' win, Barnes issued a statement offering his "sincere congratulations" to the Republican victor, saying he looked forward to a rematch of the 1978 race, which he won by less than 5,000 votes out of 155,000 votes cast.

Barnes declined to be drawn into an early campaign square-off. Asked about Steers' "rubber stamp" remark, the incumbent merely smiled and said, "I think we'll be getting into the issues as the campaign goes along. Tonight's the night to let him enjoy his victory."

Barnes, 37, was unopposed in the Democratic primary.

In the state's only other closely contested congressional primary, incumbent Democrat Clarence D. Long, 71, had no trouble holding off a challenge from 31-year-old state Del. Thomas B. Kernan in Baltimore County's 2nd Congressional District.

The crotchety Long, whose constituent work has won him a cadre of fiercely loyal supporters -- and whose opposition to plans for dredging Baltimore harbor has incurred the wrath of the city's business establishment -- will be opposed in November by Republican Helen Delich Bentley, former head of the Federal Maritime Commission.

Elsewhere in the state, incumbent Democratic Congress members had an easy time of it: Gladys Noon Spellman walked away with the victory in the 5th Congressional District in northern Prince George's County while Beverly Byron had no trouble outdistancing five challengers in the 6th Congressional District in Western Maryland.

Kevin Igoe, 31, a Treasury Department budget analyst who favors a 30 percent federal tax cut over the next three years, was leading the field in the race to challenge Spellman with more than 25 percent of the vote counted.

Byron will face Republican Raymond E. Beck, 41, a Westminster attorney and three-term state delegate, in the fall.

Republican incumbent Marjorie S. Holt, 61, had no primary opposition for the 4th District seat, which covers southern Prince George's and all of Anne Arundel County. A 44-year-old history teacher, James J. Riley, held a comfortable lead late last night in the six-man Democratic race for the right to challenge Holt this fall.

In the 1st District, the Eastern Shore's popular Republican Rep. Robert Bauman, 43, who had no primary opposition, will face a rematch with his 1976 opponent, Royden Dyson, 31, a state delegate who comes from a well-known Southern Maryland family.

In Baltimore City, incumbent Democrats Parren J. Mitchell and Barbara A. Mikulski won convincing primary victories. Early today, their opponents' primary races were still too close to call.