One of the most progressive members of Congress, Vermont’s Independent Senator Bernie Sanders, has declared he’s running for president, on the Democratic ticket.

Although Sanders is considered a long shot, given his age (he’s 73) and left-of-center political views, he’s likely to help curb the presidential debate toward certain populist issues that may well affect small business owners. Such topics include wage and income disparities, Wall Street excesses, an immigration overhaul, and the potential wrongs of free trade agreements.

"People should not underestimate me," Sanders told the Associated Press on Thursday. "I've run outside of the two-party system, defeating Democrats and Republicans, taking on big-money candidates and, you know, I think the message that has resonated in Vermont is a message that can resonate all over this country."

An avowed socialist, though in his own words not of the “authoritarian communist” kind, Sanders is so far the only Democratic challenger to Hillary Clinton, the former first lady, New York Senator and Secretary of State. Clinton declared her candidacy for president on social media earlier this month.

As such, he is filling a gap left by Elizabeth Warren, the junior senator from Massachusetts, who has said she will not seek the presidency. And he is likely to force Hillary Clinton to define her stance on issues, political experts say.

“There has been a clear demand among the most progressive and left-leaning segments of the Democratic party for someone to articulate the ways in which their agenda is distinct from [Hillary] Clinton’s,” says Robert Shapiro, a senior policy scholar at Georgetown's Center for Business and Public Policy, and former economic advisor to President Clinton.

Though Sanders defines himself as an Independent, he will be running as a Democrat. In addition to Clinton, he is likely to square off against former Maryland governor Martin O'Malley and Jim Webb, a former senator from Virginia for the Democratic ticket.

The Democrats face a rapidly filling Republican slate of candidates that currently includes Ted Cruz, the junior senator from Texas, Rand Paul, the junior senator from Kentucky, and Marco Rubio, the junior senator from Florida, and Carly Fiorina, the former chief executive of Hewlett-Packard. Jeb Bush, the former governor of Florida, and Scott Walker, the current governor of Wisconsin, are also likely to make announcements that they are seeking the presidency later this spring.

Sanders frequently strikes a strong populist note on issues such as the unfair advantages of the wealthiest one percent of earners in the U.S. and the failure of campaign finance laws to put an end to unlimited political spending by special interest groups.

Here’s what Sanders said in a Tweet from Wednesday:

He’s also been an ardent opponent of free trade agreements, such as the Trans-Pacific Partnership, a free trade agreement currently under consideration that would form an alliance between 12 nations, including the U.S., Vietnam, Peru, Australia and Japan, to limit tariffs and other barriers to trade.

Sanders’ objections stem from the long history of job losses, reported labor abuses, and environmental damage which many opponents say stem from trade deals in the past, such the North American Free Trade Agreement, hammered out under former President Clinton in the 1990s. Some small business owners have viewed NAFTA as unfairly increasing overseas competition for their products and services.

In the past couple of years, Sanders has voted against the Keystone XL Pipeline, vetoed by the president in March, as well as for bills that would have prohibited corporate interference with women’s health care and increased environmental standards for damaged national infrastructure. He also voted for comprehensive immigration reform in 2013. And Sanders co-sponsored the Senate bill that would have raised the federal minimum wage to $10.10 an hour, as well as legislation that would have provided emergency refinancing for students with high amounts of education loan debt. Neither bill got traction in the House.