Joe Manchin actually said...

I don't think it's impossible, but it would depend on a strong and assertive and self-confident American President at the time.

Either way.

So, that's why I'm afraid that, if we reject this deal, we will end up not with a better one, but with an Iran that continues down the path ...

If the United States rejects the deal now, it will not be possible to negotiate a new one.