I like Tammy a lot. I know her personally. She is a great Senator, and I am very hopeful she can ride an anti-Trump wave to reelection. Having said that, I don't think she could put together a strong enough campaign to win the 2020 nomination. And I doubt she'd run.
Do you think she can win reelection in a neutral year, and if Walker wins the gubernatorial race?
No such thing as a "neutral year" given the partisanship in Wisconsin. I would say that, given that she beat Tommy Thompson, yes, she could beat a strong GOP candidate even in a year that didn't naturally favor the GOP, but I doubt that 2018 will be a good year for the GOP in general, and Walker will have a harder time getting reelected than she will.