Of course this could scramble Trump's agenda for Sept. but that assumes incorrectly that Dems have some intention of not fighting it tooth and nail simply because it's Trump's agenda.  The real story here is that Harvey has the potential to help Trump's agenda in the long-term. The fact that the citizens of Texas are showing America what it is to come together has already made it more difficult for the dems to back anti-America groups like Antifa. Notice the same thing happened this week that has been happening for nearly a year (Antifa beat people up) and Dem leadership finally had to call them out on it. America in general is beginning to say we need to come together as a country partly because natural disasters show us the stupidity of petty differences. . 
Make Dems fight a stand alone Harvey relief bill. If they do they're toast. Thus dems will have to fight the full funding bill by itself.  If the government has to be shut down then it's not as big a deal.