In his election-night victory speech, Barack Obama said he would be a president for all Americans, not just those who voted for him. But as a candidate he didn't campaign with equal vigor for every vote. Instead, he and John McCain devoted more than 98% of their television ad spending and campaign events to just 15 states which together make up about a third of the U.S. population. Today, as the Electoral College votes are cast and counted state-by-state, we will be reminded why. It is the peculiar mechanics of that institution, designed for a different age, that leave us divided into red states, blue states and swing states. That needs to change.

The Electoral College was created in 1787 by a constitutional convention whose delegates were unconvinced that the election of the president could be entrusted to an unfiltered vote of the people, and were concerned about the division of power among the 13 states. It was antidemocratic by design.

Under the system, each state receives votes equal to the number of representatives it has in the House plus one for each of its senators. Less populated states are thus overrepresented. While this formula hasn't changed, it no longer makes a difference for the majority of states. Wyoming, with its three electoral votes, has no more influence over the selection of the president or on the positions taken by candidates than it would with one vote.

We often forget that the power to appoint electors is given to state legislatures, and it is only because they choose to hold a vote that Election Day is at all relevant for us. Nowhere is a popular election constitutionally required. And, as the 2000 election reminded us, the winner of the popular vote is not guaranteed to become president.

The Constitution is no longer in line with our expectations regarding the role of the people in selecting the president. Yet several previous attempts to eliminate the Electoral College through a constitutional amendment have failed, scuttled by the difficulty of the process itself and the tyranny of small-state logic.

Fortunately, a constitutional amendment is not necessary. Rather than dismantling the Electoral College with an amendment, we can use the mechanisms of the Electoral College itself to guarantee popular election of the president.

To understand how the proposal works, one needs to understand two basic principles. First, that state legislatures are basically unfettered in how they choose to appoint electors. And second, that groups of states can enter into binding agreements with one another in the form of so-called interstate compacts. There are many examples of such compacts, including the Port Authority of New York and New Jersey and the interstate agreement that guarantees a driver points on a Virginia driver license when he or she speeds in Maryland.

Under the proposed National Popular Vote compact, state legislatures would agree to choose electors who promise to support the winner of the nationwide popular vote. For example, if a Republican were to win the overall national popular vote, even if New Yorkers favored the Democrat, New York's Electoral College votes would go to the Republican. The compact will go into force when states representing 271 Electoral College votes have entered into it to guarantee that the winner of the popular vote will become president.

It is ironic that the most common objection to the National Popular Vote compact is the suggestion that it is antifederalist. In fact, interstate compacts lie at the very core of federalism: individual states combining their powers to solve a problem. In this case, they would be joining forces to allow their citizens to act as one nation in the selection of their president.

The National Popular Vote compact has already been enacted by four trailblazing states -- Maryland, New Jersey, Illinois and Hawaii -- and has been introduced in 41 others. It's time that the rest of them got on board.

Mr. Soros is the deputy chairman of Soros Fund Management and a supporter of the National Popular Vote.