Last in a 12-part series on the 10 amendments of the Bill of Rights.

When it came to finishing their handiwork, the Framers of the Bill of Rights may have thought they were saving the best for last. The Tenth Amendment declares: "The powers not delegated to the United States by Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." It emphasizes that the American people have given some power to the federal government and some to the states, but have reserved the rest to its original source: themselves.

To our modern ears, the Tenth Amendment states an obvious truth. But in the 18th century, it declared the radical idea that all sovereignty resides in the people, not in a monarch or the state. The people control the government, not the other way around. The Tenth Amendment reminds us that the American people established a national government of limited, enumerated powers, with states exercise their own sovereignty, and both together form our federal system of government.

Partisans of Hillary Clinton learned this to their regret in November. Expressed through the Electoral College, which forces candidates to assemble votes state by state, federalism gave Donald Trump the presidency. If the United States were a simple democracy, like many of our Western peers, a national majority would directly choose the president and the cabinet. But our Founders feared rule by the majority. They separated the powers of the national government into three branches (president, Congress, and judiciary), divided the nation's authority between the Capitol and the states, and gave the states a direct hand in the selection of the federal government. States enjoy equal representation in the Senate, which controls half of Congress' legislative power. Most other major national actions, such as ratifying treaties, appointing cabinet officers and federal judges, and amending the Constitution, must also run through the Senate.

Federalism has other benefits beyond preventing a tyranny of the majority. States remain closer to the people, which creates greater accountability in government. Pennsylvanians, for example, can monitor more easily the activities of the state assembly than Congress. They can more easily replace their governor and state legislators if they no longer represent their views. Pennsylvanians can even structure their government as they wish, such as providing for the election of judges or attorneys general, and then making it easy to remove them.

Federalism also takes advantage of decentralization. In a nation as vast as the United States, no central state could collect and analyze all the information, nor efficiently deploy the huge bureaucracy, necessary to govern a nation of 320 million people living on a continent four time zones wide. State officials are closer to the communities they serve and have better knowledge of local conditions and needs. They will have better judgment on policies affecting our every day lives, such as crime, education, the family, welfare, and infrastructure.

While Washington's powers "will be exercised principally on external objects, as war, peace, negotiation, and foreign commerce," James Madison wrote in Federalist 45, "the powers reserved to the several States will extend to all the objects, which, in the ordinary course of affairs, concern the lives, liberties, and properties of the people."

Federalism also encourages experimentation. Justice Louis Brandeis once famously called states the "laboratories of democracy." Social problems, such as economic growth, inequality, crime, health care, and education, continue to afflict the American people in all 50 states. But the right answers are anything but obvious. Federalism allows the nation to experiment with the best policies. If a state hits on a good program, as Wisconsin did with welfare reform in the late 1980s, Congress can adopt it nationally. If a state makes a disastrous choice, as California continues to do with its tax rates, other states can learn from its example.

Federalism even enhances diversity. It recognizes that people will have different preferences on social policies. Some people will favor low taxes, to promote economic growth, but at the price of reduced social services. They will move to Texas. Other people may willingly support broad spending on universities and health care, even at the expense of the economy. They will move to California. Some people like to suffer through many seasons of losing sports teams. They will stay in Philadelphia. States will compete to offer packages of economic and social policies to attract residents. Federalism creates a market for government services, much in the same way that commercial markets allow individuals to choose what products to buy.

But perhaps the most important purpose of federalism is its most radical: to check the Capitol. The Framers expected states to prevent the federal government from bursting its bounds and intruding on the rights of the people. States "will always be not only vigilant but suspicious and jealous guardians of the rights of the citizens, against encroachments from the federal government, [and] will constantly have their attention awake to the conduct of the national rulers," Alexander Hamilton wrote in Federalist 26. States "will be ready enough, if any thing improper appears, to sound the alarm to the people and not only to be the VOICE but if necessary the ARM of their discontent."

The states will always serve as the centers of opposition to D.C. They can lead by example by adopting policies unpopular inside the Beltway. They can send officials to Washington to oppose the party in power. They can even refuse to cooperate with national policies and force the federal government to fight uphill to get its way. Concerned about tyranny, the Framers erected a federal system that encouraged conflict between the Capitol and the states, even at the price of slower, less efficient government.

Republicans believed that President Obama expanded the powers of the federal government beyond its constitutional limits by nationalizing health care, accelerating regulations to light-speed, and refusing to enforce the immigration laws. They successfully mounted their resistance in the states, eventually took control of the House and Senate, won control of about two-thirds of the state governments, and have now reaped the ultimate prize: the presidency.

Democrats are coming to have similar fears about a President Trump who might persecute minorities, lift environmental protections, or eviscerate social services. They should copy a page out of the Republican playbook and return to the states to rebuild their party. If they do, they will reveal once again the wisdom of the Framers in placing federalism at the core of the Bill of Rights.

John Yoo is a University of California at Berkeley law professor, a former Justice Department official, and an American Enterprise Institute scholar. jyoo@law.berkeley.edu