Well then make it a Bernie/Warren ticket, and Trump going after Hillary.
Heck, Trump goes after Bernie directly, that costs him some Bernie voters already. And if Bernie is on the ticket, is Trump supposed to lay off him?
As it stands, we can only guess who the DEMs will run.
Warren went after Trump. Is he supposed to just take it?
There are two good reasons not to attack Warren as long as she’s not a candidate:
1) She’s a foil to Clinton. When Clinton says she will be the “first woman President”, most Dems cringe because they want Warren to have that honor. To Dems, she’s the good woman to Clinton’s evil woman. She thus by merely existing acts to undermine Clinton’s support among Democrats.
2) Most people aren’t aware of this, but the pre-politics academia version of Warren in part laid the groundwork for Trump’s success. “The Coming Collapse of the Middle Class” provides a lot of detail to buttress Trump’s arguments. It’s a fascinating watch and isn’t political at all. If Trump doesn’t destroy her he can use her as a weapon against Clinton. Check this out and start 5 minutes in: https://www.youtube.com/watch?v=akVL7QY0S8A