Sen. Bernie Sanders, an independent from Vermont, plans to enter the race for the Democratic presidential nomination on Thursday, a person familiar with his thinking said, presenting competition from the political left to front runner Hillary Clinton.

Mr. Sanders, one of the Senate’s most consistent liberals, will join Mrs. Clinton as a formal candidate for the Democratic nomination. He is expected to make a statement online Thursday and hold a kickoff event at the end of May in Burlington, Vt.

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