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Senator Bernard Sanders, the independent from Vermont, plans to run for president as a Democrat, becoming the only official party challenger so far to Hillary Rodham Clinton.

According to people familiar with the senator’s plans, he will release a statement on Thursday and make a more formal announcement of candidacy later next month in Vermont. That event will likely take place at City Hall in Burlington, where he was mayor.

Mr. Sanders, who considers himself a “Democratic Socialist,” has been traveling recently in early nominating states like Iowa, New Hampshire and South Carolina to gauge interest in his candidacy.

While considered a long-shot for the Democratic nomination, Mr. Sanders could be another voice drawing Mrs. Clinton to the left because of his progressive views on trade, health care and corporate lobbying.

“Are we prepared to take on the enormous economic and political power of the billionaire class, or do we continue to slide into economic and political oligarchy?” Mr. Sanders said on his website.

Mr. Sanders’ plans were first reported by VPR News, Vermont’s public radio station.

According to a Federal Election Commission filing, Mr. Sanders has $4.63 million on hand for his 2018 Senate re-election campaign. That money can be transferred to his coffers for a presidential race.

Amy Chozick contributed reporting.