Billionaire businessman and former New York City Mayor Michael Bloomberg's decision to re-register as a Democrat has stoked speculation about a potential presidential run challenging President Trump in 2020. But let's be clear – Bloomberg will not be the next Democratic presidential nominee.

To start, Bloomberg's decision to officially return to the Democratic Party already highlights his political opportunism. Though he was always a Democrat, Bloomberg ran as a Republican in 2001 because the Democratic mayoral field was crowded and Republicans, desperate with Rudy Giuliani facing term limits, were happy to clear the way for a self-funded candidate. When he got what he needed from the GOP, he eventually declared himself an independent and now is returning back to the Democrats. Nevertheless, in an environment in which Democratic primary voters will be looking for somebody who they can count on to fight on their side, Bloomberg's wavering is going to hurt him.

On top of this, as mayor, Bloomberg continued many of Giuliani's anti-crime strategies and has been a staunch defender of "stop and frisk" including in this Washington Post op-ed. With racial disparities in policing a significant issue for Democrats, and with black voters being a crucial part of the Democratic primary electorate, it's impossible to see how he can be competitive.

Furthermore, with Democrats increasingly concerned with income inequality, angry at Wall Street, and eager to run a class warfare campaign, a Wall Street billionaire without a populist bone in his body would not seem to have much of a base.

The Jewish Michael Bloomberg's previous support for Israel – even in-person praise for Benjamin Netanyahu – is going to be a problem for him, and I have no doubt we'll see whisper campaigns and social media posts raising questions about dual loyalty that will further undermine his chances as anti-Semitism grows on the Left,.

It's really hard to see what his constituency would be outside of a handful of columnists in New York and Washington.

[Also read: Here comes Joe? Biden signals he will run in 2020]