Sen. Bernie Sanders (I-Vt.), a potential White House contender in 2016, is known for his stances on budget issues and war. Here's the Vermont senator's take on Obamacare, Social Security and more, in his own words. (Julie Percha/The Washington Post)

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 generated little interest from rank-and-file party members. Others on the Democratic radar — former Maryland governor Martin O’Malley and former Virginia senator Jim Webb — have not formally decided on running.

Sen. Bernie Sanders (I-Vt.) will launch a bid for the Democratic presidential nomination on Thursday. (Win McNamee/Getty Images)

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 ties to Wall Street.

Sanders’s backers said they hope he can serve as a proxy for Warren’s disappointed drafters, helping to animate small-dollar Democratic donors with his brash persona and speeches condemning the “billionaire class.”

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 because of 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.

The senator’s political adviser will be Tad Devine, one of the Democratic Party’s leading consultants and a former high-level campaign aide to Al Gore, John F. Kerry and Michael Dukakis.

“He is not only a longtime client but a friend,” Devine said in an interview last year. “I believe he could deliver an enormously powerful message that the country is waiting to hear right now and do it in a way that succeeds.”

In recent months, Devine and Sanders — who first worked together in the 1990s — have been mapping out how the brusque senator could navigate the race and present a challenge to Clinton.

Sanders, a Brooklyn native, is the longest-serving independent in congressional history. Before winning election to the Senate in 2006, he served in the House and as mayor of Burlington.

Sanders, who caucuses with Democrats in the Senate, has seen his profile rise since 2010, when he delivered a marathon filibuster on economic policy. That speech turned into a book, and Sanders has since appeared frequently on MSNBC and HBO.

Jose A. DelReal contributed to this report.