Recently, there's been a debate among some liberal writers about the problems with the term "soft power," which lends itself to misunderstanding as being "soft" in the sense of weak and ineffectual. Others suggest "smart power," which may well be a better term for political leaders to use, and still others prefer "principled power."
If we set aside the question of what's a good slogan and instead consider what's a good conception of power, then I think "principled power" is on the right track. It implies not just that power ought to be used in a principled way, but that the power of a liberal society grows directly out of its principles -- first of all, out of its principles of constitutional constraint (the idea that constitutionally limited power can be more powerful than unlimited power); and second, out of its principles of equality and social inclusion.
The experience of recent years ought to reinforce those convictions. Bush hasn't trespassed constitutional limits with marvelous results; he's degraded American power in both senses of the word "degrade." And a more inclusive conception of our interests (inclusive internationally as well as at home) has a far greater chance to achieve the ends Americans want. This is why I argue in Freedom's Power for a liberalism that takes power (as well as rights and justice) seriously. Among the various conceits of conservatism is that it alone knows power, when it's actually failed on the very ground it sees as its own. There has been no better time to make that case than this election year.