WASHINGTON — Vermont Sen. Bernie Sanders, an independent and avowed socialist, formally launched his campaign for the presidency Thursday, promising to make the economic unease of America’s middle class the driving force of his longshot bid.

In an era in which running for the nation’s highest office has become a vehicle for self-promotion for some, Sanders’ kickoff was a classic underdog pitch to draw greater attention to his chief issues, economic inequality and a political system increasingly tilted to the well-off.

“This campaign is not about Bernie Sanders,” the rumpled 73-year-old said at a news conference outside the Capitol. “It is about a grassroots movement of Americans standing up and saying: ‘Enough is enough. This country and our government belong to all of us, not just a handful of billionaires.'”

“The major issue is, how do we create an economy that works for all of our people rather than a small number of billionaires?” he added.

Sanders, the longest-serving independent in the history of Congress, will campaign for the Democratic Party’s nomination, which he said gives him a better platform to engage in debates and mobilize support.

“The reality is that if you want to engage in debates, if you want to mobilize people, it is hard to do it outside the two-party system,” he told CBS News.

His entry makes him a leading potential challenger to party heavyweight Hillary Rodham Clinton, if not for votes and campaign donations then for pressing her to maintain fidelity to progressive causes.

In Congress, Sanders has advocated for higher taxes on the wealthy and increasing the minimum wage and is a lead opponent of President Obama’s proposed trade deal among Pacific nations. In 2010, Sanders controlled the Senate floor for more than eight hours in opposition to an extension of lower tax rates.

Sanders also voted against the 2002 authorization that paved the way for the Iraq war. Clinton’s vote for that resolution figured prominently in her 2008 campaign against Obama.

A national Quinnipiac University poll conducted in mid-April showed Clinton leading the race for the Democratic nomination with 60 perccent support from primary voters. Sanders polled at 8 percent. But recent surveys from Public Policy Polling, a Democratic-leaning firm, showed Sanders cracking double-digit support in Iowa and New Hampshire.

Rep. Debbie Wasserman Schultz, D-Fla., the Democratic Party chair, welcomed Sanders to the contest, saying the senator is “well-recognized for his principled leadership and has consistently stood up for middle-class families.”

Sanders made no mention of Clinton in his remarks to reporters or in an email announcement to supporters. “This is not the Red Sox versus the Yankees,” the New England lawmaker vying against the New York-based Clinton campaign told reporters.

“In a democracy, what elections are about are serious debates over serious issues. Not political gossip, not making campaigns into soap operas,” he said.