When you speak up in favor of something or support a cause, that's espousal. Your espousal of energy conservation can be seen in your electric car and the solar panels on your house.
Espousal of one political candidate over another can be expressed with a sign in your yard or a big financial contribution to the campaign. During a debate, each person adopts the espousal of one strong viewpoint and then they argue with each other. In the old days, an espousal was an engagement or a wedding — gaining a spouse, in other words. This is the original meaning, with the figurative "married to principles" definition dating from about 1670.