Top White House and GOP leadership officials tell us the chances of a market-rattling government shutdown are rising by the day — and were even before Trump threatened at his raucous Phoenix rally on Tuesday night to use a shutdown as leverage to get funding for a border wall. Trump is dead serious about this fight, a senior administration source tells us, and the president's talk is starting to spook markets.

Goldman Sachs, in guidance to investors last Friday, pegged the odds at 50/50. This strikes us as high, but the dynamics are ominous:

This strikes us as high, but the dynamics are ominous: A top Republican source put the chance as high as 75%: "The peculiar part is that almost everyone I talk to on the Hill agrees that it is more likely than not."

"The peculiar part is that almost everyone I talk to on the Hill agrees that it is more likely than not." This may all come down to Trump's mood: The president is spoiling for a fight and the [conservative House] Freedom Caucus haven't had a fight for a while. That's a dangerous dynamic.

Based on funding mechanisms, the showdown could come either in September or December — or both:

Officials at both ends of Pennsylvania Avenue who are up to their necks in tax reform think passage probably doesn't happen until early next year.

who are up to their necks in tax reform think passage probably doesn't happen until early next year. So a September shutdown could be better for tax reform than a Christmas shutdown, because it would allow conservatives and Trump to get it out of their system.

Democrats feel certain they have Trump boxed in, and see no reason to compromise/help:

Trump is at war with Senate Leader McConnell and several other Republicans, complicating communications and compromising trust.

with Senate Leader McConnell and several other Republicans, complicating communications and compromising trust. Congressional leadership doesn't want a shutdown and can pass the fall bare necessities — continuing resolution / debt ceiling / Children's Health Insurance Program extension — using mostly Democratic votes.

doesn't want a shutdown and can pass the fall bare necessities — continuing resolution / debt ceiling / Children's Health Insurance Program extension — using mostly Democratic votes. But the Freedom Caucus will hammer Speaker Ryan for doing so, and conservatives in the Senate will hammer Leader McConnell.

Be smart: With the departure of Steve Bannon, Trump is surrounded more and more by conventional/mainstream folks, which could actually make him feel more compelled to buck them.