Sen. Ted Cruz (R-TX)
If Congress is going to prevent the government from shutting down, the House will need to pass a spending bill on Wednesday so that the Senate can move more quickly than the Senate ever does and pass the bill on Thursday. That means absolutely no procedural delays in an institution with a million possible procedural delays. One of those may be particularly relevant: Getting the bill to a final vote in time will
require the cooperation of every single senator, since even one senator can force two cloture votes and 30 hours of debate, which would go past the Thursday night deadline.
Sahil Kapur suggests that both Sens. Ted Cruz and Jeff Sessions might go this far, but the Washington Post reports Sessions saying "You’re concerned about what the president did and want to respond—but you don’t want to be slowing things down to the point that a near-shutdown occurs." So he's probably out, but that still leaves Ted Cruz, who has not been sounding real reasonable or conciliatory.
One possible workaround would be for the House to pass a short-term continuing resolution lasting for a few days to avert a temporary lapse in funding. That could be passed in the Senate with one cloture vote, rather than two, but even that would be subject to a debate that blows past the Thursday deadline.
Senate Democrats are hoping Cruz doesn't go there.
"[A]t that point, he's shutting down the government for a day, which seems insane even for him," one senior Senate Democratic aide said.
I'm not sure it's ever safe to bet against Ted Cruz being extremist, but between Senate rules and Cruz being Cruz, hoping he won't be "insane even for him" may be all there is to do.