Here is a comment from Ambassador Lighthizer in response to a question from Congressman Panetta at last week's House Ways and Means Committee hearing (starts at 3:23:09), on the choice between NAFTA or USMCA:
I think we have clearly made an enormous, probably enormous, improvement in this bill, and I just, every time I talk to I know you and others, I always think to myself at some point you’re gonna have to decide whether there’s this horrible agreement that you think is so bad that you’ve campaigned against it since you were in high school, although I wasn’t in high school, you were in high school, and/or this improvement. I keep saying to myself, you know, you can at some point, you gotta say okay fine we’re just gonna get rid of the uncertainty we’re gonna just vote for the improvement and not say, “ah, but we could even make it better and better and better,” so that’s a little bit of venting and frustration on your time.
Lighthizer's answer here suggests that members of Congress face a choice between the original NAFTA and the USMCA. But it seems to me that Congressional Democrats might see a third option: A revised NAFTA negotiated by a future Democratic president. I would imagine that once the Democrats get settled on a presidential nominee, they will be pretty fired up and expecting to win the 2020 election. With that in mind, would they prefer to vote for a trade deal negotiated by Trump, a vote they will have to defend for the rest of their careers? Or would they rather put all of this on hold until after the election, after which the Democratic president can negotiate a deal that is more to their liking?