WARSAW — A political crisis in Poland entered a new and dangerous phase on Wednesday, as the country’s Constitutional Tribunal struck down a new law limiting its powers. The nation’s right-wing government, which adopted the law in December, said it would ignore the court’s ruling.

The legal showdown is the latest manifestation of a rightward lurch that began after the conservative Law and Justice Party swept to power in elections in October.

The government is facing growing pressure from the European Union and the United States to respect the decisions of the Constitutional Tribunal, the only court empowered to review national laws. In January, the European Commission, the executive body of the European Union, opened an unprecedented inquiry into whether the government had violated democratic norms, a condition of membership in the 28-nation bloc.

Poland, which has the sixth-largest economy in the European Union, is a major net recipient of economic development aid. It is also a member of NATO and an American ally against an increasingly belligerent Russia.