[SKempis]

It is right there - could not be more simple. Unless the 3rd party is a serious threat to the Dem, if you would have given your vote to the conservative, but did not, and instead took that vote and gave it to a third party, you helped the Democrat by giving him one less vote he must match against his real threat.

Now, instead of your conservative vote cancelling Joe Blow's Dem vote, making the balance equal (con=+1, Dem=+1), Joe Blow's vote gives the dem advantage (con=0 Dem=+1), whatever the final outcome.

[Sloth]

if you would have given your vote to the conservative

There is the problem. Why is it assumed that the lesser of two evils is somehow entitled to my vote, like some sort of default candidate selection, so that if I vote for someone else I am 'taking away' support for that so-called second choice? Is not the person I actually VOTE for entitled to that vote?

Was Al Gore entitled to Ralph Nader's votes? Perhaps you think they should have been counted toward his total, and Buchanan's toward Bush?