"At key points in U.S. history, one of the two parties has served as a bulwark against those who threaten our Constitution. Two years ago at the Democratic Convention, I warned of those threats. Today, I have re-registered as a Democrat -- I had been a member for most of my life -- because we need Democrats to provide the checks and balance our nation so badly needs."

You'll remember (or maybe not!) that Bloomberg was a Democrat for much of his adult life, before leaving the party to register as a Republican in 2001. That decision was an entirely political one; he wanted to be mayor of New York City, and the Republican Party -- and primary -- offered a much more direct route to that goal.

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Bloomberg won that race -- thanks to his massive personal spending -- and got re-elected in 2005 as a Republican, again. Two years later, however, Bloomberg left the GOP and registered as an unaffiliated voter. At the time, he explained the move this way : "Any successful elected executive knows that real results are more important than partisan battles and that good ideas should take precedence over rigid adherence to any particular political ideology. Working together, there's no limit to what we can do."

That gambit was seen by political observers as Bloomberg setting the stage for an independent run for president in 2008, a campaign that never materialized after Barack Obama and John McCain won their respective party nominations. Bloomberg got elected to a third term as mayor of New York City in 2009 as an independent and then, four years later, left elected office.

Which brings us to Wednesday -- and Bloomberg's latest party chameleon shift. It's no big secret that Bloomberg is (and has long been) interested in running for national office. And his decision to speak at the 2016 Democratic National Convention seemed to be a pretty clear indicator of where his party loyalties currently stood.

So what does it then tell us, then, that Bloomberg thinks his best potential path to that goal is as a Democrat?

1. The idea of a viable third party is dead.

I've long been an on-the-record skeptic of the idea that someone running outside of the two-party system could be anything but a spoiler for one side or the other in a presidential contest.

Yes, lots and lots of people point out that the fastest-growing political party in the country is "unaffiliated." But lots and lots of people who ID as independents aren't actually independents. When rubber meets road, they tend to consistently side with the same party. People like the idea of saying they are independent but very few of them are actually up for grabs for the two parties in any meaningful way. I think that's even more true in the era of Trump; there are just not a whole lot of people who are in the political middle (or anywhere close to it) these days.

Then there are the huge logistical hurdles that come with trying to run outside the two-party system. The two party structures do so much of the less glamorous work of running for president -- collecting signatures to ensure the party's candidate appears on the ballot in each state, for one -- that is not only time consuming but also very, very expensive. Bloomberg is a billionaire -- and could potentially spend his way out of that problem. But it would take tens of millions -- and that's before a single ad is bought or a visit to an early voting state is planned.

2. The Democratic primary race is totally wide open.

Did you know there are more than 30 Democrats actively mentioned as considering a run for president in 2020? Thirty! That massive field tells you that a) there is no clear front-runner in the race and b) there is a whole hell of a lot of pent-up ambition among Democrats who, by and large, passed on the 2016 presidential race. Bloomberg looks at this field and, judging by his decision to re-register as a Democrat, isn't scared by the Joe Bidens, Bernie Sanders and Elizabeth Warrens of the world.

There's some reason -- emphasis on some -- for Bloomberg's confidence. Early polling of the ginormous field suggests that no one is running away with the nomination as it appeared Hillary Clinton was doing at this time in the 2016 cycle. And Bloomberg does have oodles of money (of which he has shown a clear willingness to part with for political causes in which he believes) and a record as mayor of New York City that Democrats, generally speaking, would applaud. ( Bloomberg's support for "stop and frisk" policing policies are a notable exception .)

And did I mention Bloomberg has oodles of money?

3. The outsider businessman angle isn't played out.

There's an argument that after four years of a billionaire businessman with no prior experience outside of New York City, Democrats -- and the country --aren't going to nominate someone with a profile anything close to that of Trump. Bloomberg clearly believes that is not the case. The argument he would be making is not that a successful businessman can't run the country but that Trump is neither all that successful nor all that much of a businessman.

Bloomberg's theory of the 2020 case was made plain during his 2016 DNC speech:

"Throughout his career, Trump has left behind a well-documented record of bankruptcies, thousands of lawsuits, angry shareholders, and contractors who feel cheated, and disillusioned customers who feel ripped off. Trump says he wants to run the nation like he's run his business. God help us. ... I understand the appeal of a businessman president. But Trump's business plan is a disaster in the making."

You can see this pitch from Bloomberg in Iowa or New Hampshire sometime in 2019: I'm the actual billionaire businessman from New York City. We've tried an incompetent one, now we need to let a competent one get into the Oval Office.

Is Bloomberg right about that message -- or his broader assessment of running for president as a Democrat in two years' time? I'm genuinely not sure. But I'm more convinced today than I was yesterday that we're going to get a chance to find out.