Apart from the Donald Trump sideshow, one of the biggest stories in the presidential election this past week has been the apparent momentum Sen. Bernie Sanders (I-Vermont) is enjoying in his underdog campaign.

Sanders has been drawing crowds of thousands at his rallies and is quickly becoming the main primary rival of Democratic frontrunner Hillary Clinton. I totally get Bernie Mania. However, I’m deeply conflicted about it.

My wife works for Hillary so there’s that. But I’m also torn because I don’t really understand what he is doing.

I served with Bernie and he is my kind of politician — a progressive guy with some New York City attitude. It’s hard not to love Bernie Sanders. The Brooklyn accent perfected at Madison High School and Brooklyn College and the rumpled mad scientist look are perfect compliments to his colorful and unyielding presentations.

Still, I have one major question for Bernie. What exactly does he think he’s doing in a Democratic presidential primary? Why is he asking for the nomination of a party he always avoided joining?

While we were together in Congress, if I was ever homesick for some five borough flavor, I didn’t need to get on Amtrak and have one of my constituents yell at me on Queens Boulevard. I could just light a Bernie bomb by prodding him about his party affiliation. The query always got a typically bombastic reaction out of Bernie.

Senator Bernie Sanders (I-VT) addresses International Association of Fire Fighters (IAFF) delegates at the IAFF Presidential Forum in Washington March 10, 2015. Reuters Independent Bernie Sanders seemed to like this question. He probably got it a lot. He would tell me that I shouldn’t confuse the fact that our voting records generally matched with party agreement. He was a proud socialist and thought the institutional Democratic Party was too cautious and lacking imagination. As much as I prodded, I would never get him to think about joining the Democrats for a moment.

In fact, Bernie always got me fired up to make the fighting wing of the Democratic Party feistier. So much so, that I loved it when my less clever right wing opponents would decry Obamacare as “socialism”. Bernie and I would remind these blockheads that giving people tax credits to buy a product from a giant corporation is hardly socialist.

There’s no question Bernie’s leftist agitating is filling a void in this primary process. The Democratic Party has a strong primal scream element right now. It expresses itself in frustration that the high expectations of change that came with President Obama have not been met. It howls at the failure of candidates who hew to the middle of the road and it feels the need to counter the batshit crazy it sees dominating the debate on the other side of the aisle.

Our party needs a kick in the butt. Sen. Elizabeth Warren (D-Massachusetts), Paul Krugman, and Jon Stewart are currently the standard bearers for that sentiment.

But Bernie Sanders? I just don’t know.

After a career of steadfastly insisting that the Democratic party was not his home, now he wants to not only be a member of the party but its standard bearer? What changed?

Is Bernie’s newfound party affiliation just a practical decision to run in a party that can win rather than risk being a Nader-esque spoiler on a third party line in November? That’s a fair calculation, but doesn’t it wipe away Bernie’s three decades of standing as a principled Socialist?

Many times over the course of his career Bernie has repeated the line that his independence made him more able to speak truth. He argued forcefully that being a Socialist was his identity and not function of political expediency. Well, duh, nobody chooses to be a Socialist to smooth their political path. Yet, as 2016 approaches, here he is filing papers all over the country presumably declaring himself a member of the Democratic Party.

Bernie is right about a lot of things. He is right that a Medicare for All health care program is a simpler, cheaper and more American solution to our health care needs than a jury rigged system that is better under Obamacare but still has too many gaps. And his battle cry on behalf of working Americans is almost as good as Hillary Clinton’s.

In spite of all this, if Bernie wants to lead this party, he needs to explain what he's doing here in the first place.