Retrenching after the Nuclear Regulatory Communistion's decision Friday halting construction of the controversial Seabrook nuclear power plant, pro-nuclear formes here yesterday called for legal and congressional action to reverse the NRC ruling.

Wearing a hardhat and speaking from the back of a flatbed truck. Gov. Meldrim Thomson told a cheering crowd of about 500 workers at the partially built $2.3 billion Seabrook plant yesterday mornings that the state would file a halt-billion dollar suit against the federal government to recover unemployment benefit costs "if they dare to close this placed to close this place own."

However, state Attorney General Thomas Rath later said a more likely route for the state would be to join in legal action by either the Public Service Company builders of the plant, or the workers labor union to head off the July 21 deadline for work stoppage at the plant.

New Hampshire Building and Construction Trades Council president Joseph Monarty said his group which is threatened by mass layoffs if the plant is shut down, plans to file suit in Washington to reverse the NRC decision.

Later in the day, the New Hampshire congressional delegation told a hastily mustered pronuclear rally here - dubbed an "Emergency Energy Congress" - it will throw its weight behind an administration bill to cut the average 12-year licensing time on nuclear power plants.

Firing barbs at the Nuclear Regulatory Commission, the congressmen proposed amending Department of Energy and NRC appropriations bills to assure construction of the plant.

The move was spurred by a call-to-arms against the NRC decision by Thomson, who has vowed to make Seabrook the rallying cry of a pronuclear power grass roots movement, which he hopes will have a far greater constituency than California's tax-cutting Proposition 13. He is seeking re-election to a fourth term.

Despite shouts of "no" by a small but ardent band of antinuclear protesters at the "Energy Congress," the group approved a nonbinding resolution supporting federal legislation to assure the continuance of "full-scale construction activity at Seabrook station."

To head off the July 21 deadline - a move the congressional delegation agreed is impossible through legislation - Thomas called on the 2,200 Seabrook workers and the Public Service Company to appeal the NRC decision.

Company officials said the construction delay will cost about $15 million a month, and 1,800 workers will be laid off.

Picking up Thomson's battle cry, Sen. Thomas J. McIntyre (D-N.H.), who is seeking reelection to a fourth term charged the NRC with "vacillation, nitpicking and indecisivenes."

Calling for a shakeup in the federal bureaucracy, Rep. Norman D'Amours (D-N.H.) said. "We have to bring them kicking and screauring into the 20th century technological revolution."

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Construction of the Seabrook plant - the site of several anti-nuclear power demonstrations and a national symbol of the struggle against nuclear power - has been suspended eight times since it was unveiled about 10 years ago.

Opponents of the plant claim it is potentially devastating to human and marine life. Its advocates contend it is essential to providing electrical needs for this region - one of the hardest hit by the energy crunch.

The state's political leaders urged the crowd to launch a letter-writing campaign to President Carter. Energy Secretary James Schlesinger and Environmental Protection Agency administrator Douglas Costle supporting a quick reversal of the NRC decision on the Seabrook plant.