Do you really need an affirmation that Trump would be the popular choice by supporting a primary challenge that would most likely fail?
Your primary challenge might have a chance if Democrats voted in it. I just don't see enough turnover in the Republican party for another candidate to be able to defeat Trump in a primary.
Trump has lost supporters. Even a few here. That's anecdotally factual. Demographers will tell you that process scales up. Not as many people - Dems and Republicans - support him as in 2016. Because polling with regards to Trump has (IMHO) been unreliable, there's no clear way to tell what percentage.
So, the only way to test it is to test it.
It isn't even just a Republican party turnover issue. Trump attracted big numbers ("YUGE" in his words) of Dems in order to get elected. How many of them do you think still support him? (Actually, quite a few, probably, given the socialist nature of the current Dem candidates.)
Look, odds are Trump will win the primary. But, the odds were against him last time, so that doesn't really mean much.
If you want Trump to lose the primaries, you have to be doing your best to make sure Bill Maar get's his recession.
Of course that's another R word, so likely a fail the same as Russia and Racist...
Economic corrections are inevitable. I haven't blamed presidents for those since the 1980s (when I got "woke" to how the economy works). (I'm not sure I used that right.) So, same goes for Trump.
The tariff issue isn't great for our economy, but that's just part of the overall picture. The tax reform is at least somewhat mitigating of that problem.
From my perspective, Trump is a terrible president. I only want him to lose the primary to a better candidate. Just like I would only want him to lose the general to a better candidate.
Right now, I'm not sure there is a better candidate. Truly, better the devil you know than the devil you don't.