WASHINGTON — Sen. Bernie Sanders, I-Vt., a blunt self-described socialist who has become a favorite of progressive activists for his denunciations of big banks and the financial elite, will jump into the 2016 presidential campaign on Thursday, according to two people familiar with his plans.

One ally – who spoke on the condition of anonymity to discuss Sanders’s timetable – said the 73-year-old senator is expected to make his intentions known this week and hold a rally in Vermont next month. He plans to run as a Democrat, according to his associates.

The decision to get in the contest was first reported Tuesday by Vermont Public Radio.

Sanders presents a notable left-leaning challenge to Hillary Rodham Clinton, the front-runner for the Democratic presidential nomination, who announced her second campaign for the White House on April 12.

Former Rhode Island governor and ex-Republican Lincoln Chafee announced his pursuit of the Democratic nod this month, but his campaign has so far generated little interest from rank-and-file party members. Others on the Democratic radar – former Maryland Gov. Martin O’Malley and former Virginia Sen. Jim Webb – have not formally decided on running.

Sanders shares many of the same political stances as Sen. Elizabeth Warren, D-Mass., a darling of liberals who has repeatedly said she is not running for president. That means Sanders may end up serving as the most prominent voice for the left wing of the party, particularly voters who are suspicious of Clinton and her close ties to Wall Street.

Speaking not long ago with The Washington Post, Sanders said his message would be concentrated on the “collapse of the middle class” and “income and wealth inequality,” which he called a “huge issue from a moral sense and a political sense.”

Sanders chose to run in the Democratic primary due to his interest in participating in the party’s primary debates, according to confidants. If he ran as an independent, he would not be able to engage with the national Democratic infrastructure or act as a direct foil to Clinton in the early primaries and caucuses.