For those who don't have a Bible at hand, Psalm 109:8 reads in the NKJV, "Let his days be few, and let another take his place." That's enough of a twist on Christian ethics, since the Bible also commands Christians to pray rightly - as in, pray for their success - for leaders placed over them. The wider context makes this particularly ugly. I was aware of that, but a lot of the people putting up these billboards and the politicians who have invoked it casually may not.
The next several verses, again in the NKJV:
9Let his children be fatherless,
And his wife a widow.
10 Let his children continually be vagabonds, and beg;
Let them seek their bread[b] also from their desolate places.
11 Let the creditor seize all that he has,
And let strangers plunder his labor.
12 Let there be none to extend mercy to him,
Nor let there be any to favor his fatherless children.
13 Let his posterity be cut off,
And in the generation following let their name be blotted out.
That's enough of a downer. "In the generation following let their name be blotted out" basically means that his line won't continue - that any sons will either not have sons themselves, or else will die young. Not particularly pro-life. And even without that, it's hard to imagine the people who laugh at this being a prayer they can say for Obama laughing at the idea of his children starving in the wilderness. (Recall the context - it would've been next to impossible for a widowed, disgraced woman to earn a living.)
BUt it actually takes a turn for the absurd, and almost becomes funny, when you take into consideration the real context. As Robert Cargill points out, this is David recounting the curses other people have made against him:
1Do not keep silent,
O God of my praise!
2 For the mouth of the wicked and the mouth of the deceitful
Have opened against me;
They have spoken against me with a lying tongue.
3 They have also surrounded me with words of hatred,
And fought against me without a cause.
4 In return for my love they are my accusers,
But I give myself to prayer.
5 Thus they have rewarded me evil for good,
And hatred for my love.
Help me, O Lord my God!
Oh, save me according to Your mercy,
27 That they may know that this is Your hand—
That You, Lord, have done it!
28 Let them curse, but You bless;
When they arise, let them be ashamed,
But let Your servant rejoice.
29 Let my accusers be clothed with shame,
And let them cover themselves with their own disgrace as with a mantle.
So just to recount, you have a godly king recounting people wanting him to die and see his family in ruin, and then praying that God will help him withstand their curses "that they may know that this is Your hand" when he is saved from those curses.
I'm setting aside the question of what relationships religion and politics should have. I personally think anyone who wants their particular vision of God to overwhelm the political process is being a bad American, but that's a side point. Invoking this kind of prayer, in light of the Biblical command (1 Timothy 2) to pray for our leaders means you're being a bad Christian. And using this particular reference to do it would be laugh-worthy, if the thought behind the prayer wasn't so awful.
I'd say if God has anything to do with this whole affair he's having a good belly-laugh at the irony. Of course, I'm pretty sure God doesn't enter into this whole line of "prayer."