This Thursday, Sen. Bernie Sanders (I-VT) will announce that he's running for the Democratic presidential nomination, according to multiple reports . Sanders served as mayor of Burlington, Vermont, for eight years and as his state's sole member of the House of Representatives for 16 years, and has been in the Senate since 2007. Through all of that time, he has been an independent — though, in Congress, he caucused with the Democratic Party. Sanders will be the second prominent candidate to join the Democratic race — and will take on Hillary Clinton, the frontrunner. He'll focus heavily on issues of inequality and rebuilding the middle class, and his opposition to the Trans-Pacific Partnership will be a major part of his campaign, reports VPR's Bob Kinzel . Campaign finance reform is sure to be another area of serious emphasis.

Why Sanders is running as a Democrat

Last September, I visited Iowa with Sanders as he weighed a presidential bid. At a town hall event in Waterloo, one audience member urged him to run, to cheers and applause. Another then asked whether, if he did enter the race, he'd run as an independent or a Democrat. "That's a great question!" Sanders said, animated. "I'd love to get your opinions on it."

"the media will take you more seriously"

He laid out his thinking to the crowd. An independent candidacy could be appealing because of "huge frustration at both parties." But the drawback was, he said, that it's very difficult to get on the ballot in 50 states. And he emphasized that he would never run as a spoiler if it could lead to the election of a Republican president — "we've made that mistake in the past."

On the other hand, if he ran as a Democrat, he said, "It's easier to get on the ballot, you can get into the debates, and the media will take you more seriously." The disadvantage? "People are not overwhelmingly enthusiastic about the Democratic Party."

Sanders asked the crowd which sounded better, and about 80 percent of them raised their hands in favor of a primary contest. "I think you run as a Democrat, because you want to push the debate, with Hillary or whoever it is, in the direction you want to see it go," an audience member said. "We need to hear the establishment challenged."

The senator then asked the crowd another question. "I know Iowa does politics differently than other states," he said, to knowing chuckles. "How many of you would be prepared to work hard if I ran?" A sizable majority raised their hands again. Read more in my profile, "Bernie Sanders vs. the billionaires."

WATCH: The role of third parties in politics