There's a risk they'll try to break the Senate rules outright [0], by pretending that certain promised-to-be-temporary tax cuts now cost $0 to extend.
To put it in domestic terms:
* [January 1st] "Honey, I want to rent a Ferrari, I did the math and it fits if it's just one month! Pleeeeeease?"
* [February 1st] "Oh, that? It's the Ferrari rental-fee for the next month, don't worry, it's an existing expense, it's already part of our regular budget, so clearly we've proven we can afford it. We'll just have to cut back on insulin for the kids."
[0] https://www.americanprogress.org/article/senate-republicans-...
Both parties do this to make spending bills appear smaller. This is why clean energy tax credits generally passed during Democrat administrations have to be periodically renewed.