TOKYO — Proposed new safety guidelines for Japan’s nuclear industry — strict enough that they could keep reactors shuttered for years for emergency upgrades — have set off intense political maneuvering by those who say the regulations will cripple business just as hopes were rising for economic relief.

The relatively stiff requirements by a panel that included nuclear power supporters appeared to take Japan’s nuclear industry and its backers in government by surprise, and pose a challenge to Prime Minister Shinzo Abe just weeks into his term.

Mr. Abe has made it clear that he wanted to restart Japan’s scores of idled reactors — all but two of which remain offline in the wake of the 2011 Fukushima plant disaster — and has even said that he wanted to build new ones. But he and his Liberal Democratic Party, the architect of Japan’s nuclear industry, already faced significant opposition from a population that was traumatized by the nuclear crisis that spread radioactive materials over a wide swath of the country’s northeast.

The crisis at the Fukushima plant, which led to meltdowns in three reactors, started after a 9.0 magnitude earthquake caused a tsunami that swept through the plant, knocking out the electrical power needed to run crucial cooling systems.