Making official what was initially reported on Tuesday, Sen. Bernie Sanders has told the Associated Press: "I am running for president."

In an interview with the news agency, Sanders jumped right into a discussion about his populist mindset and progressive agenda while issuing an additional message for those doubting that he—a self-described democratic-socialist—can compete in a national campaign against the early Democratic frontrunner Hillary Clinton or take on the Republican Party's field of candidates.

"People should not underestimate me," Sanders told the AP. "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."

Regardless of his field of potential opponents, Sanders said that "the question is whose views come closer to representing the vast majority of working people in this country. And you know what? I think my views do."

Though Sanders has talked publicly about the possibility of a run since last year, cheers from across the progressive community went up this week when the news of his decision broke.

"Bernie Sanders says he's running for president to win," exclaimed fellow Vermonter and 350.org co-founder Bill McKibben, who called the news "an extremely cool thought!"

On Thursday morning, a campaign website went up at BernieSanders.com and the new candidate and his team have been blasting out messages via Twitter offering insights into both his platform and campaign strategy. For example:

A strong grass roots movement... pic.twitter.com/X7zyf3NoOp — Bernie Sanders (@SenSanders) April 29, 2015

According to the AP:

The son of an immigrant from Poland who sold paint for a living in Brooklyn, Sanders, 73, has for decades championed working-class Americans. He lost several statewide races in the 1970s before he was elected mayor of Burlington in 1981, and went on to represent Vermont in the U.S. House for 16 years before his election to the Senate in 2006. An independent, he caucuses with Senate Democrats and is likely to attract some interest from voters who have unsuccessfully sought to draft Massachusetts Sen. Elizabeth Warren to join the race. But he rejected the idea that his appeal is limited to voters on the left, boldly predicting that his message would appeal to independents and Republicans. Sanders said he would release "very specific proposals" to raise taxes on wealthy Americans and corporations, as well as offer tuition-free education at all public colleges and universities. He touched on his past opposition to free-trade agreements, his support for heavier regulations of Wall Street and the nation's banking industry, and his vote against the Keystone XL oil pipeline as a preview of his campaign.

Making it clear that themes of economic inequality and the role of big money in politics will be central to his campaign, Sanders tweeted a message on Wednesday saying that in this presidential race, every candidate must be able to answer the following questions:

Is it morally appropriate that the 99% of all new income is going to the top 1%?

Is it good economics that the top one-tenth of 1% own almost as much wealth as the bottom 90%?

Is our democracy being destroyed when one family can spend $900 million to buy election?

"What we have seen is that while the average person is working longer hours for lower wages, we have seen a huge increase in income and wealth inequality, which is now reaching obscene levels," Sanders told the AP. "This is a rigged economy, which works for the rich and the powerful, and is not working for ordinary Americans. ... You know, this country just does not belong to a handful of billionaires."

Despite the scourge of money in politics, Sanders is not naive to the fact that a viable presidential candidate must raise vast sums of money to compete at the national level. That the last presidential election cost nearly $3 billion and the 2016 cycle is predicted to be a $5 billion spend-fest, Sanders called "a real disgrace," saying the role of dark money and Super PACs in a post-Citizens United world represents the "undermining of American democracy" and is one of the main reasons he has decided to run.

But, he added, "Can we raise the hundreds of millions of dollars that we need, primarily through small campaign contributions to run a strong campaign? And I have concluded that I think there is a real chance that we can do that."

The question of whether or not Sanders should be taken "seriously" or not by the establishment press in Washington, D.C. or New York City was tackled head-on by Rolling Stone journalist Matt Taibbi on Wednesday, who wrote a column chastising the insular and all-too-often myopic political reporting that has come to dominate election seasons, particularly presidential campaigns. Taibbi scoffed at the idea that Sanders needs to prove his viability as a candidate to the Beltway press corps, writing:

It's a little-known fact, but we reporters could successfully sell Sanders or Elizabeth Warren or any other populist candidate as a serious contender for the White House if we wanted to. Hell, we told Americans it was okay to vote for George Bush, a man who moves his lips when he reads. But the lapdog mentality is deeply ingrained and most Beltway scribes prefer to wait for a signal from above before they agree to take anyone not sitting atop a mountain of cash seriously. Thus this whole question of "seriousness" – which will dominate coverage of the Sanders campaign – should really be read as a profound indictment of our political system, which is now so openly an oligarchy that any politician who doesn't have the blessing of the bosses is marginalized before he or she steps into the ring.

And as journalist Glenn Greenwald tweeted, "Before media figures decree Bernie Sanders a loser running a futile campaign, maybe they can let Americans hear what he believes/advocates."

In December of last year, Sanders put forth what he called an Economic Agenda for America, a 12-point plan which would:

Invest in our crumbling infrastructure with a major program to create jobs by rebuilding roads, bridges, water systems, waste water plants, airports, railroads and schools. Transform energy systems away from fossil fuels to create jobs while beginning to reverse global warming and make the planet habitable for future generations. Develop new economic models to support workers in the United States instead of giving tax breaks to corporations which ship jobs to low-wage countries overseas. Make it easier for workers to join unions and bargain for higher wages and benefits. Raise the federal minimum wage from $7.25 an hour so no one who works 40 hours a week will live in poverty. Provide equal pay for women workers who now make 78 percent of what male counterparts make. Reform trade policies that have shuttered more than 60,000 factories and cost more than 4.9 million decent-paying manufacturing jobs. Make college affordable and provide affordable child care to restore America’s competitive edge compared to other nations. Break up big banks. The six largest banks now have assets equivalent to 61 percent of our gross domestic product, over $9.8 trillion. They underwrite more than half the mortgages in the country and issue more than two-thirds of all credit cards. Join the rest of the industrialized world with a Medicare-for-all health care system that provides better care at less cost. Expand Social Security, Medicare, Medicaid and nutrition programs. Reform the tax code based on wage earners’ ability to pay and eliminate loopholes that let profitable corporations stash profits overseas and pay no U.S. federal income taxes.

Writing for The Nation this week, journalist and political commentator John Nichols described Sanders' bid as a "Which side are you on?" campaign.

"As a presidential candidate," explained Nichols, "he can force the debate that the Democratic Party, and America, needs to have... on a long list of issues that the 'billionaire class' would have preferred to keep off the table."

Taibbi signed off his post with a word of congratulations to Sanders for stepping into the ring. "Good luck," he wrote, "and give 'em hell."