Lisa Marshall Manheim is an associate professor at the University of Washington School of Law who specializes in constitutional law, election law, and the federal courts. She is the co-author of "The Limits of Presidential Power: A Citizen's Guide to the Law." Follow her on Twitter @ManheimAndWatts. The views expressed here are solely hers.

(CNN) Donald Trump, as a presidential candidate, never promised compromise. He instead promised the opposite: reform through sheer force of will. "Nobody knows the system better than me," said Trump when accepting the Republican nomination, "which is why I alone can fix it." The trouble with such a self-centered approach? It is contrary to the Constitution.

Lisa Marshall Manheim

Indeed, it is our nation's powerful legal framework that explains so much of what President Trump has accomplished, and failed to accomplish, during his first year in office. This framework sustains a system of government that resists unilateral action and rejects singular efforts to consolidate power.

But past performance is no guarantee of future results. And as President Trump's second year begins on Saturday, it remains to be seen whether these constitutional constraints will survive.

For now, the rule of law is still standing, and this durability bodes well for its future. But the rule of law cannot be taken for granted. Indeed, it is under attack -- by the same man it is constraining.

To understand the significance of legal constraints, take, for example, Trump's promise to build a "big, beautiful" wall separating the United States from Mexico. A year in, construction sites remain empty. This is because the Constitution allows the federal government to spend money -- including the billions it would take to build a border wall -- only if Congress authorizes that spending. Thus far, Congress has refused to deliver.