Who should a Christian vote for?

How do I discern who to vote for politically? I have many friends who are on both sides (Republican & Democrat). How do we weigh politicians against our beliefs, say in the case of abortion or homosexuality?

There is no "right" way for a Christian to vote, so every believer should follow their conscience in the matter. All human beings are sinful (including all political candidates), so there is no "righteous" choice available. Only Christ is perfect, and when He arrives to rule the earth, then righteousness will reign:

Is. 9:6 For a child will be born to us, a son will be given to us;
   And the government will rest on His shoulders;
   And His name will be called Wonderful Counselor, Mighty God,
   Eternal Father, Prince of Peace.
Is. 9:7 There will be no end to the increase of His government or of peace,
   On the throne of David and over his kingdom,
   To establish it and to uphold it with justice and righteousness
   From then on and forevermore.
   The zeal of the LORD of hosts will accomplish this.

Until then, the world will be ruled by imperfect men and women, therefore there is no clear advantage in voting for one candidate over another.

Moreover, a Christian's true allegiance should not be to an earthly country but to our true “country” in Heaven, according to Scripture:

Heb. 11:13 All these died in faith, without receiving the promises, but having seen them and having welcomed them from a distance, and having confessed that they were strangers and exiles on the earth.
Heb. 11:14 For those who say such things make it clear that they are seeking a country of their own.
Heb. 11:15 And indeed if they had been thinking of that country from which they went out, they would have had opportunity to return.
Heb. 11:16 But as it is, they desire a better country, that is, a heavenly one. Therefore God is not ashamed to be called their God; for He has prepared a city for them.

Those who believe in the Gospel understand that a new world is coming soon, and this new world is our true home. In the meantime, we are wanderers in a foreign land (i.e., on this Earth). Paul says we are ambassadors (2 Corinthians 5:20), which means we are not part of this world. We are representatives of a different world living in a foreign land.

Therefore, our focus should be on preparing ourselves and our world for the arrival of the true King of righteousness, not on finding a "righteous" choice among our present, corrupt leaders. In short, vote your conscience.