Mark Noonan over at Battle Born Politics has a wonderful essay on why this question is both disingenuous and impractical. Among other things, Noonan writes,
Leaving aside the fact that even if support for the war is at 33% then there are 10 million military-aged males in the United States who support the war (and, thus, no room for even 20% of them in the military), what the left is doing is revealing its own, totalitarian mindset - that liberal concept, first thought up in the French Revolution, that the State has absolute claim on our lives and property in a time of national emergency. No conservative can actually support such a view - even at times of national emergency, people have right to not participate - that isn't a libertarian position (though libertarians hold to it, as well), but an understanding that while society does have a claim on us, its claim is not absolute..
Noonan also points out that the all-volunteer force we currently posses is better trained, better equipped and better motivated than any conscript force could possibly be and that a surfeit of warm bodies encourages commanders to be profligate with those bodies, as we saw in the World War I slaughters on the Western Front. Noonan says that human lives are too valuable to waste. I absolutely agree.
Noonan's fellow blogger Matt Margolis also wrote a post on his own personal views on the question of joining the Armed Forces. Read the whole thing.
In conclusion, leftists and liberals have very little understanding of the makeup of the United States Armed Forces and virtually no expertise in using these forces. If a liberal asks the question, you can try to educate him or her, though the odds are that he or she is too submerged in their fever swamp of choice to actually listen. For the Left, Truth has long been superceded by Desire of What Truth Should Be.