I'm slamming macro below. Even then, I think we know a fair amount, as is implied by my hedging. Economists are useless when it comes to many fundamental questions important to policy, not all. I think micro counts as a science, especially empirically-focused experimental micro. My rhetoric is overbroad, so my apologies to empirically-oriented micro people.
In another excellent Economix post that I missed (thanks Gherald L), Princeton's Uwe Reinhardt makes a version of my point, showing how easy it is for macroeconomists to cherrypick estimates of, say, the responsiveness of workers to changes in marginal tax rates to support what they've already concluded. The moral of Reinhardt's story deserves emphasis:
So there you have the flexibility, shall we say, that economists enjoy when they apply their professional skills to affairs of state in what may seem, to outsiders, like purely scientific analyses.
In the first lecture of my freshman economics course at Princeton titled “The Art of Siffing Among Seasoned Adults,” I demonstrate how seasoned adults routinely structure information felicitously(i.e., “sif”) to further their own agenda, and I point out that economists can be among the most skillful practitioners of this art.
“If at the end of this course you still trust me,” I warn them, “I have failed in my mission. When economists advise on public policy, the operative mantra is Caveat Emptor!”
I am sad to teach it, but consider it fair and full disclosure.
Will Wilkinson 
To the extent such campaigns do anything, it results in the opposite of what it’s trying to accomplish, further ghettoizing male liberalism with the kind of effete, supplicant males females really don’t like to choose. Among men who have a lot of sex partners, Republicanism and chauvinism are rampant, in part, because it serves as an honest signal of their capability of attracting women. Lesser men have to refashion their politics to please young single women, while attractive men don’t need to. I suspect the hierarchy of male desirability looks like this:
Unattractive Pubs – Least desirable
Unattractive Dems – 2nd least desirable
Attractive Dems – 2nd most desirable
Attractive Pubs – Most desirable
And while men are more likely to tweak their politics to win over sexual partners, women are even more likely to conform their politics to their long term partner. So while these ‘vote Dem, get laid’ posters are rather cute, they better realize that it sets a precedent, and that Republicans could actually turn it around and cut even deeper, by suggesting that Democrat girls are just whores for sex and fun, while Republican girls are the valuable ones you invest in long term.