If one must waste their vote by voting for a major party candidate instead of a third party candidate, there is no law saying that one must do so in an unintelligent manner. It still can pay to be smart and think ahead.
Most people who vote major party do not do that. Comments such as "I hope Bachmann wins because that would mean an easy victory for Obama" are common enough. The problem with such a wish is that it can too easily backfire. People saying such a thing should remember that whoever wins the Republican nomination does stand a good chance of winning the presidency.
It is therefore much smarter to hope that the best person wins the Republican nomination instead of the worst person. And if both parties are having a primary, as happened in 2008, it is better to hope that the best person in each race wins so that no matter which party wins the ultimate winner is better than he would otherwise be.
Although there are plenty of short-sighted reasons for the Democrats to support various candidates, the list of candidates that a Democrat could positively support is much shorter. It comes down to two candidates; Mitt Romney and Ron Paul. Which candidate the Democrat should choose depends on whether the Democrat is a liberal or a progressive.
Progressives, if thinking strategically, would prefer Governor Romney. He is typical of a "moderate Republican" in that he supports the worst aspects of both the Republican agenda and the Democrat agenda.
Liberals, on the other hand, have every reason to support Representative Paul. He wants to end the wars in the Middle East. He would achieve the greatest advancement in civil rights since granting minorities the right to vote by ending the war on drugs. Although he is pro life, he would do more for guaranteeing the right to choose by sending the issue back to the states than either Romney or Obama. But most importantly he will end the wars, bring the troops home, and stop wasting money spent on these adventures, money that is currently being taken from the poor through inflation.
Democrats can influence who will be the Republican nominee, and if they honestly assess what they stand for they should admit that Ron Paul actually is the best in the Republican field. An honest answer should be "if we have to have a Republican as President, then it might as well be Ron Paul." For that reason they should help him win instead of participating in the blackout.