Politics and Thinking Things Through
This kind of argument is always ridiculous, because it's based on the premise that people who disagree with you are morons who intentionally want to lose.
They can see why it would be disastrous as well as you can. So either they won't do it, or one of you is wrong about how disastrous it will be. And you have no basis for assuming that you're correct and they're wrong.
People always act in whatever way they perceive to be in their best interest. If your theory denies this, either by assuming people will intentionally act against their interests for no reason, or by failing to explain why their perception of what's in their interest is mistaken, then your theory is wrong.
A couple common examples of this phenomenon are:
1. Republicans who assume that Hillary Clinton will mount a primary challenge against President Obama, which will split the Democrats and hand the Republicans an easy electoral victory.
2. Democrats who assume Republicans will nominate Sarah Palin for President in 2012, which will hand Obama an easy electoral victory.
If you believe either of these things will happen, why do you think the people on the other side can't see what's obvious to you? If your answer is that all the people who disagree with you are too dumb to think things through, then it's really you who is failing to think things through.
Also, please don't respond to this by pointing out past political moves that seem dumb in hindsight, such as the Republicans nominating Christine O'Donnell for the Delaware Senate race. If the Republicans had known how gaffe-prone she was and the kooky stuff that would come out about her, they would have picked someone who had a better chance to win. But it's not stupid to lack access to a time machine. The Republican primary voters in Delaware made the decision that they believed would best promote their interests based on the information they had at the time. They ultimately turned out to be wrong, but they weren't *trying* to be wrong.