Here is a comparison of what the leading candidates say about marriage on their "issues" pages of their campaign web sites. These are just excerpts but I have provided the links if you want to read all of what they say. I assume that these issue statements were carefully decided upon and carefully crafted to appeal and communicate to a national audience (as opposed to a speech directed to a particular special interest audience). I have added some comments in red.
Clinton: This topic did not rise to the level of being included in her official issue statements.
Obama: This topic did not rise to the level of being included in his official issue statements.
McCain: The family represents the foundation of Western Civilization and civil society and John McCain believes the institution of marriage is a union between one man and one woman. It is only this definition that sufficiently recognizes the vital and unique role played by mothers and fathers in the raising of children, and the role of the family in shaping, stabilizing, and strengthening communities and our nation.
Huckabee: [As usual, Huckabee is direct, succinct, and specific about where he stands on this issue.] I support and have consistently supported passage of a federal constitutional amendment that defines marriage as a union between one man and one woman.
Other recent comparison posts of mine: abortion, stem cell research, immigration, energy/environment, federal spending