WASHINGTON — President Obama is preparing regulations limiting carbon dioxide emissions from existing power plants, senior officials said Wednesday. The move would be the most consequential climate policy step he could take and one likely to provoke legal challenges from Republicans and some industries.

Electric power plants are the largest single source of global warming pollution in the country, responsible for nearly 40 percent of greenhouse gas emissions. With sweeping climate legislation effectively dead in Congress, the decision on existing power plants — which a 2007 Supreme Court decision gave to the executive branch — has been among the most closely watched of Mr. Obama’s second term.

The administration has already begun steps to restrict climate-altering emissions from any newly built power plants, but imposing carbon standards on the existing utility fleet would be vastly more costly and contentious.

The president is preparing to move soon because rules as complex as those applying to power plants can take years to complete. Experts say that if Mr. Obama hopes to have a new set of greenhouse gas standards for utilities in place before he leaves office he needs to begin before the end of this year.