Six Senate liberals on Thursday are holding the inaugural meeting for the newly formed Senate Social Security Caucus.
The huddle comes on the same day that a trio of Senate conservatives met to form the Senate Tea Party Caucus, providing a stark contrast between the two sides in the heavily divided upper chamber.
Sanders drew attention to the contrast between his group and the Tea Party Caucus after the meeting.
"As Bernie convened the new Defend Social Security Caucus, the new Tea Party Caucus met down the hall. Which would you join? #caucus," his office wrote in a tweet.
Democrats have long used Social Security as a political cudgel against Republicans, arguing that the party wants to privatize the popular entitlement program.
Republicans have argued that Democrats are not serious about reforming Social Security, which is expected to be insolvent by 2037, and say that all options should be on the table.
According to a release from Sanders's office, the group will meet with organizations "representing senior citizens and workers to discuss the future of Social Security."
Representatives from the AFL-CIO, the Service Employees International Union, the American Federation of State, County and Municipal Employees, and Democratic pollster Celinda Lake were also expected to attend.
-- This post was updated at 1:41 p.m.