Congress regularly passes significant laws. Some of these laws continue in their initial form, with the original bargain struck by the enacting coalition untouched by any future laws; others are changed—strengthened or weakened—soon after passage. What accounts for this variation in the stability of laws, in the longevity of the original legislative agreement? We contend that political conditions at the time of enactment—in particular, the existence of divided government and the level of ideological disagreement between the House and Senate—influence the likelihood that a law will be amended. We demonstrate that laws originally crafted by diverse political coalitions are less durable than those crafted by strong, unified coalitions, which are in a position to entrench their preferred policies and protect them from future change. Furthermore, we show that the probability of a law being amended is affected by future political conditions, the actions of the judiciary, and factors specific to the law.