U.S. Sen. Bernie Sanders, I-Vt., speaks to low-wage federal contract workers demanding presidential action to win a $15-an-hour wage on Dec. 4, 2014, in Washington, D.C. (Photo: Win McNamee/Getty)

Vermont Senator Bernie Sanders — a self-avowed socialist who caucuses with Democrats — is set to announce his presidential bid Thursday, becoming the first left-leaning candidate to formally launch a challenge against Democratic front-runner Hillary Clinton.

And though Sanders looks more like Back to the Future’s Doc Brown than a seasoned political veteran, his progressive but pragmatic record in Congress suggests that he could be a formidable force within the 2016 Democratic primary. A 16-year member of the U.S. House before joining the Senate in 2007, Sanders has built a large audience for his political viewpoints outside of Congress while forging unlikely bipartisan relationships within it to pass major bills with top Republicans.

Along the way, Sanders has built a substantial following on social media, thanks in large measure to his focus on the economic inequality issues that animate and rally the American left.

While Elizabeth Warren has gained breathless media attention as the top progressive alternative to business-friendly Democrats, Sanders has gone under the Beltway radar to become a liberal icon on platforms like Facebook, where with more than 970,000 “likes,” his official page has more followers than those of other official Senate pages (though other senators have more on their campaign-side sites). Sanders’ Twitter profile has nearly 100,000 more followers than Warren’s. And his official Senate.gov website is the most popular page within that domain, according to the Web traffic data tracker site Alexa.

Unlike Warren, however, Sanders is actually entering the presidential race. He has not been afraid to challenge Clinton’s policy views and seems certain to be willing to confront her directly in presidential primary debates and candidate forums.

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Sanders also now has a more powerful Senate perch than ever before, which will allow him to run a two-pronged race — from the halls of Congress and from the road.

This year, Sanders became the top Democrat on the Senate Budget Committee, giving him a megaphone and the staff resources to churn out liberal economic proposals aimed at the lower and middle classes in a framework that many Senate aides expect will be more liberal than previous Senate Democratic budget plans. Even without taking the step of formally entering the race, being the top Senate Democrat on the influential economic panel gives Sanders some leeway to force conversations within the party on issues such as entitlement reform and social safety-net programs.

Sanders speaks out against the Republican-backed budget in Washington on April 29, 2015, saying it will hurt American middle-class families. He is joined by, from left, Sen. Charles Schumer, D-N.Y., Rep. Steve Israel, D-N.Y., and Rep. Chris Van Hollen, D-Md. (Photo: Carolyn Kaster/AP)

With the freedom to create a budget in his own image and talk about it frequently, Sanders might not get endorsements from Senate colleagues who are inclined to back Clinton, but he is likely to get their support and praise for his work on this essential political bread-and-butter policy portfolio. The budget debate is likely to happen soon, setting up Sanders to get prime C-SPAN and cable news time as Clinton is still going through the motions of her soft campaign launch. Moreover, as the top member of the minority party, much of Sanders’ actual responsibility is being the top attack dog against the Republican budget, a role he seems adept at and eager to play.

“The budget passed last year by the Republican House — which called for massive cuts in Medicare, Medicaid, education, nutrition, affordable housing, and other programs impacting the lives of working Americans while providing huge tax breaks for the wealthy and large corporations — is a budget approach which moves us in exactly the wrong direction,” Sanders said in January at the first of his many press conferences this year as the panel’s ranking member.

“When we prepare a budget we have got to look at the reality of American life, and we build that budget based on that reality. … While the economy has made significant gains in the last six years, the simple truth is that the American middle class has been declining over the last forty years. Today, at a time when the wealthy and large corporations are doing phenomenally well, median family income is nearly $5,000 less than it was in 1999.”

It’s easy to map what Sanders will say as a candidate because he’s been so consistent in his messaging as a senator, from an eight-hour 2010 floor speech against President Barack Obama’s extension of the George W. Bush-era tax cuts (“Filibernie,” as the pseudo-filibuster was called on Twitter, became the sixth-most-popular trending topic on the service) to his constant press releases from the budget panel, where he has vowed to pressure colleagues by using social media and his connections to various interest groups from previous committee work.

But unlike his counterparts on the far right and despite his heated political rhetoric, Sanders has not disrupted the proceedings of the Senate and has been willing to work in a bipartisan way on major legislation, including a 2014 deal on a veterans’ care bill he brokered with Republican John McCain of Arizona.

The legislation, which Sanders stewarded through Congress as chairman of the Veterans Affairs Committee, was aimed at helping to fix the serious, systemic problems within the Veterans Affairs health care system. It authorized leases for 26 new medical facilities in 17 states, designated funds to hire more doctors and nurses and expanded the VA’s authority to refer patients for civilian care. It also instituted changes to the VA’s management and bureaucracy in the wake of a scandal over appointment waiting times, giving the VA the power to fire or demote the agency’s leadership staff and eliminating the use of wait times as part of employee performance measures.

Senate Veterans’ Affairs Committee Chairman Bernie Sanders, Ind.-Vt., and House Veterans Affairs Committee Chairman Jeff Miller, R-Fla., walk to a news conference in Washington on July 28, 2014, where they announced that the two committees had struck a deal to reform the Veterans Affairs Department. (Photo: Chip Somodevilla/Getty)

At the time, the legislation was not all Sanders wanted it to be, but he recognized the need to pass some reform rather than none at all.

“I would have written a very, very different bill,” Sanders said in June 2014 when his deal with McCain was announced. “Right now we have a crisis on our hands, and its imperative that we will deal with that crisis.”

It’s this sort of legislative history and ability to compromise that separates Sanders from someone like Republican Ted Cruz of Texas, another 2016 hopeful, and could prove challenging for Clinton.

For as much as Sanders can disagree with leaders on substantive and sometimes boring policy issues (like the “chained CPI,” a wonky term for indexing spending and taxes), he has never shut down the government or completely halted his own leadership over his beliefs. And in that restraint comes some political power, especially among a Democratic base that values governing and is not nearly as skeptical about the role of government as our nation as grassroots Republican voters.

Sanders also has a much more substantial legislative history than the three first-term Republican senators who’ve announced that they will seek their party’s 2016 nomination so far: In his time in the House and Senate, 206 bills that Sanders has sponsored or co-sponsored have become law, according to the Library of Congress . A number of the bills addressed veterans’ issues, including the Veterans’ Cost of Living Adjustment Act of 2013, which increased veterans’ disability compensation.