Republican Helen Delich Bentley, a tart-tongued former Nixon administration official who has doggedly pursued the 2nd District congressional seat of Maryland Democrat Clarence D. Long since 1980, latched onto President Reagan's coattails this time around and rode them to a narrow victory.

However, Bentley, with 51 percent of the vote, refused to proclaim herself the winner before her cheering supporters. And Long, who made no public appearance, refused to concede defeat because of a record number of absentee ballots -- between 6,000 and 7,000 -- that will not be counted until Thursday. Only 5,752 votes separated the contestants last night out of more than 210,000 cast, according to complete returns.

"The race isn't over till it's over," a Long campaign aide told campaign workers just before midnight. And Bentley said in an interview that the winner "won't be known until Thursday."

While the lead in the Bentlet-Long battle seesawed earlier in the night, incumbents in Maryland's seven other congressional districts walked to easy victories, most of them over lesser known and poorly financed opponents. The winners included Democrat Michael D. Barnes in the 8th District, who piled up 70 percent of the final vote, according to final results, and Democrat Steny H. Hoyer, who had 72 percent of the final vote in the 5th.

But clear-cut victory in the venomous and expensive 2nd District contest, where the conservative Bentley had wrapped herself in the Reagan mantle, eluded both contestants last night.

The 60-year-old Bentley undoubtedly was helped by the strong Reagan showing in Baltimore County, where he led the Democratic ticket by a 3-to-2 margin, and in Harford County, where he led 2 to 1. The district includes most of Baltimore County and part of Harford.

The GOP had hoped to add one more Republican to the state's eight-member congressional delegation, which now has only one, and GOP leaders put their money on Bentley. The National Republican Congressional Committee poured money into the race and sent big-name Republicans, including former president Gerald Ford and Vice President Bush to stump for her.

Long, 75, an 11-term veteran of the House and an institution in Maryland politics, fought back, outspending Bentley 2 to 1 in what appears to be the most expensive congressional contest in Maryland history. A political maverick who has built a national constituency from his position as chairman of a powerful foreign aid subcommittee, Long raised out-of-state money from Jewish supporters who know him as a friend of Israel. Together he and Bentley spent more than $1.2 million on the race.

The race, unmatched in its nastiness, developed into a classic liberal-conservative showdown over jobs, taxes, defense and social issues, and the opponents were still trading charges in the campaign's final hours yesterday.

In the 6th District, three-term Rep. Beverly Byron, a conservative Democrat, easily defeated former state Del. Robin Ficker, a political gadfly who has run for various offices as a Republican, a Democrat and an independent. He generated little enthusiam this time, even among his current chosen compatriots, the Republicans, in the district that includes Western Maryland and parts of Howard and Montgomery Counties.

In the 4th District, Rep. Marjorie Holt, the lone Republican in the present congressional delegation, easily turned aside a challenge by Democratic businessman and novice politician Howard Greenebaum. Holt, who was reelected to her seventh term, announced to her supporters that she had a new granddaughter as of 4 p.m. yesterday. "I'm going to keep the seat until she's ready to run for Congress," she told the cheering throng at her headquarters.

In the 8th District, which includes most of Montgomery County, the three-term Barnes was never really threatened by Republican Albert J. Ceccone or Libertarian candidate Samuel Grove. Ceccone's charges that Barnes was "too liberal" failed to make even a small dent in Barnes' popularity with his elite Montgomery County constituents.

In the 5th District, which includes most of Prince George's County, two-term incumbent Hoyer faced a spirited challenge from Republican John C. Ritchie, but Hoyer's solid political organization, forged from support in the black and ethnic communities, made him virtually invincible.

Rep. Barbara A. Mikulski, whose 3rd District includes parts of Baltimore, Baltimore County and Howard County, was the easy victor against challenger Ross Pierpont, a GOP physician and perennial candidate.

In the sprawling 1st District, which includes Maryland's Eastern and Western shores, conservative Democrat Roy Dyson defeated Republican Harlan Williams.

In the 7th District, Democratic Rep. Parren J. Mitchell of Baltimore was unopposed.