Markets are already excited. This looks like a "buy the rumor, sell the news" event (at least for a 25 bp cut). I don't think we'll see a crash, but with markets at all-time highs and poor seasonality in August/September, I wouldn't be surprised if there's a moderate correction after the July 31 decision.
I personally think the Fed might cut starting with 25 bps, not because of Trump's pressure but because they are seeing strong signs of an imminent recession commencing in Q1 or Q2 of next year. Just look at important economic indicators such as the Leading Economic Index, Real Retail Sales, Aggregate Hours Worked - the writing is on the wall.
Maybe that's a contrary indicator...he hasn't performed that well recently. And his economic writing, which used to be pretty good, has become more contradictory and rambling IMO.
In the central banking / academic economist world, the current belief in vogue is that CBs were behind the curve in both 2000 and 2007, and the ECB likewise reacted too slowly to Europe's double-dip in 2011-12. Right now, asset prices are holding up but there are clear signs of economic weakness globally, as well as int'l dollar funding stress, plus added risks from the trade war. OTOH inflation is somewhat below the Fed's target (unemployment figures notwithstanding) and there's no clear credit or investment bubble with potential macro significance. Thus, it's really not unreasonable for the Fed to cut a quarter-point to keep markets calm and see if things stabilize. Don't see any reason to cut 50bps, but you never know.