Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) will officially launch a bid for the White House on Thursday.

A Democratic source close to the senator confirmed to The Hill that Sanders will announce his candidacy for the Democratic presidential nomination in a statement later this week, before heading back to his home state for a campaign kick-off event early next month.

ADVERTISEMENT

Sanders will be the first Democrat to formally challenge Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE for the Democratic nomination, but he faces long odds.

The former secretary of State has a huge lead over the field of potential Democratic contenders, taking 62 percent support in the latest Fox News poll released last week.

Sen. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (D-Mass.), who insists that she’s not running for president, came in second at 12 percent, followed by Vice President Biden at 9 percent and Sanders at 4 percent.

Still, many Democrats will be happy to see Clinton get at least a token challenger. They believe it could keep Clinton sharp as she prepares for the general election, and Sanders is a liberal favorite who will challenge her on policy issues from the left.

Vermont Public Radio first reported his decision.

—This post was updated at 5:15 p.m.