WASHINGTON — U.S. nuclear power regulators are scaling back tougher safety rules proposed in the wake of Japan’s Fukushima nuclear disaster, saying additional measures imposed after the 2011 incident are sufficient.

In a 3-2 decision split along partisan lines, the Republican-majority Nuclear Regulatory Commission on Thursday voted to strike language from a proposed new regulation that would have required nuclear plants to plan for earthquakes, floods and other disasters of an intensity unimagined before the Japanese catastrophe, which was triggered by a tsunami.