All spending bill start in the House. What if their idea of spending is not approved by the Senate? Or POTUS vetoes?
Two possibilities –
(1) They pass nothing, thereby "shutting down the government," which D's hate to do (will blame it on R's or Trump, but D's don't want to shut anything down, so …)
(2) Pass a temporary spending bill, which will probably be an omnibus (that's what they've been doing for years).
Omnibus means POTUS can spend the money any way he wants.
Seriously, what are big spending D's gonna do when their big spending bills are blocked?