Colorado voters have decided to join a growing list of states that hope to decide a president by popular vote, the latest move in a national chess match over the way the United States elects its presidents.

Called the National Popular Vote Interstate Compact, the agreement calls for states to award their electoral votes to the winner of the national popular vote, once enough states join the agreement.

So far, 15 states and the District of Columbia have approved the pact, covering 196 electoral votes of the required 270 to win the presidency.

That 270 matters: The states that have approved legislation to join the compact agreed it would not take effect until the 270 threshold is reached. Once it does, those states will have the power to use their Electoral College votes to elect a winner, according to the popular vote. This uses the Electoral College to effectively phase out the Electoral College, though it would not apply to states that do not sign up.