I would argue that voting for a Repub or Dem candidate is essentially a non vote because they are so close in ideology on the role of the federal government and international policy.
Do you think president Al Gore wouldn't have invaded Afghanistan after 9/11 had he defeated Bush? Gore was also a hawk on Iraq and believed they had a solid wmd program in the works as were many prominent Dems of the period that since have since pretended they didn't have their opinions before the Bush admin....it's quite conceivable he would have invaded Iraq like Bush.
Now we have President Obama that put the Bush foreign policy on steroids, at least Bush got a vote from congress before he started a war where there was no United States interests at stake. Likewise, Bush created the largest entitlement program since the Johnson administration with his prescription drug program and Barry just upped him with his Obamacare program.
There used to be difference between the political parties but we would need a parliamentary system to end this ridiculous cycle of Repub and Dem..they're basically two wings of the same political party.