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In his 2013 inauguration speech, President Obama called for greater equality in American society. He specifically discussed the struggle of homosexual couples to achieve legitimate marriage status in the eyes of the state. Obama will likely revisit this theme in the State of the Union Address on Tuesday evening and call for states to recognize same-sex marriages as a civil right.

To understand the case for recognizing same-sex marriages, it is important to understand the legal, not religious, concept of marriage in the United States. A marriage is only legal with a marriage license, and only the individual state in which the marriage takes place can issue one. A legitimate marriage can be established through two channels. It can be purely civil, meaning a judge or other officiant certifies the marriage, or religious, meaning a religious leader both performs the marriage ceremony and officiates the marriage in the eyes of the state.

A state may grant full or partial marriage rights under a different term than marriage, namely as a civil union, a domestic partnership, or a reciprocal beneficiary relationship. When a state does recognize a marriage, the couple is able to take advantage of the multitude of rights that married status entails, such as visiting and power of attorney rights.

Individual states and not the federal government determine and recognize the legitimacy of any marriage. Therefore, while President Obama may support same-sex marriage in the State of the Union address, he cannot directly effect its legality on a national scale. Obama will likely defend same-sex marriage within the context of the Proposition 8 and Defence of Marriage Act legal cases. Because these cases are in the hands of the judicial branch, what Obama or the Department of Justice can do to influence them is very limited.

Regaardless, if the president addresses both cases and argues in favor of same-sex marriage, the gesture could strengthen social and cultural support for it. Since statewide ballots are the most common method for legalizing same-sex unions, Obama’s rally could significantly aid the cause of equal marriage rights for homosexuals by highlighting and promoting the issue.

Could Obama do something more drastic? Technically, yes. The federal government has been able to override states’ laws in indirect ways. For example, state governments set the age at which a person may purchase, possess, and drink alcohol in their respective states. If they so pleased, they could allow people under 21 to purchase and possess alcohol. However, the federal government effectively thwarted states’ ability to do so with the National Minimum Drinking Age Act of 1984. Though it does not set a minimum age for the consumption of alcohol, the act requires all states to raise the minimum age for the purchase and public possession of alcohol to 21.

If a state chooses not to comply, it will lose significant federal highway funds granted under the Federal Highway Aid Act. With the National Minimum Drinking Age Act of 1984, the federal government did not outlaw underage drinking directly and encroach on states’ jurisdiction; it instead made the penalty for states that allowed the purchase and possession of alcohol by someone under 21 so dire that each state was forced to comply. The effect remains clear today.

Such a strategy is a heavy-handed approach that the federal government can take to ensure that states align with its policies. It’s unlikely, though, that Obama would, given the political cost, or could, given the makeup of the House, attempt to promote such a strategy to same-sex marriage.

More likely, during the State of the Union, Obama will deliver a passionate defense of homosexuals’ right to marry, and going forward, the DOJ will work quietly behind the scenes to encourage in whatever limited way it can the legality of same-sex marriage.