President Obama’s smooth path to the Democratic nomination may have gotten rockier Monday, after a group of liberal leaders, including former presidential candidate Ralph Nader, announced plans to challenge the incumbent in primaries next year.

The group said the goal is to offer up a handful of candidates from various fields and areas where the president either has failed to stake out a “progressive” position or where he has “drifted toward the corporatist right.”

“Without debates by challengers inside the Democratic Party’s presidential primaries, the liberal/majoritarian agenda will be muted and ignored,” Mr. Nader said in a news release. “The one-man Democratic primaries will be dull, repetitive, and draining of both voter enthusiasm and real bright lines between the two parties that excite voters.”

In search of candidates, Mr. Nader and the others sent out a letter, endorsed by 45 “distinguished leaders,”to elected officials, civic leaders, academics and members of the progressive community who specialize among other things in labor, poverty, military and foreign policy. The list, they said, also includes progressive Democrats who have held national and state office and have fought for progressive reforms.