Because signing your name to a document automatically makes you a better candidate and person:

(Michele) Bachmann’s latest attention-getting move came Thursday when she became the first – and thus far only – candidate to sign a pledge drawn up by a Christian conservative group in Iowa. The pledge, called “The Marriage Vow: A Declaration of Dependence Upon Marriage and Family,” requires signers to promise, among other things, to keep marital fidelity, support the Defense of Marriage Act and “robust childbearing and reproduction,” and oppose gay marriage, abortion, “Sharia Islam,” “intrusively intimate commingling among attracteds” in the military, and the abuse of women and children through human trafficking, pornography and prostitution. The group, called the Family Leader, run by Bob Vander Plaats, a former Iowa gubernatorial candidate and influential conservative, promises to withhold political support from any candidate who does not sign.

Sadly, she will get votes because of this ridiculous action on an even more ridiculous document. Forcing a candidate to sign something or else withholding political support — sounds like blackmail to me. Full document here.