It seems to me that a major part of a politician’s job is to persuade. Persuasion can be done in many ways. From appealing to high ideals, to noting enlightened self interest, to threats and intimidation. (And yes, some, such as the current administration, just point-blank lie.)

Part of persuasion is to convince one’s own base that one has done all one can to win over the opposing side. It also can test the willingness of members on the opposing side to commit to conflict.

This is particularly the case where the next step might include a costly and lengthy conflict. Doing this helps prepare one’s base to remain and support the conflict.

I read Tim Kaine’s response as part of this first step. He is laying the groundwork for continuing forward to the conflict, if needed.