To denounce a candidate is to say that you shouldn’t vote for them.

Or you can vote against them.

To vote for a candidate who you say doesn’t deserve a vote is self-contradiction.

They deserve a vote solely for the reason that doing so is the only possible means of voting against the other candidate. It’s not a self-contradiction.

A tactically correct action is an action that best furthers your goals.

What are the goals in this scenario?