Voters in the US don't directly elect the president. Sometimes that can undermine the popular will

The U.S. has a unique system for electing a president, the Electoral College. In modern times, it has put disproportionate voting power in the hands of a few states that are fairly evenly divided politically.

That forces campaigns to dedicate most of their money to the so-called battleground states. There are seven of them this year — Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

The lack of attention to other states leaves voters in much of the country feeling as if they and the issues they care about are being overlooked during the presidential contest.

American voters don't choose their president directly through the popular vote. When they cast their ballot, they are technically voting for a slate of electors who will then vote for president and vice president on a specific day in December.

Nearly all states have laws binding electors to vote for the winner of their state’s popular vote, but that doesn't mean the presidential candidate who gets the most Electoral College votes is the one favored by the majority of voters.

In two of the last six U.S. presidential elections, candidates have lost the nationwide popular vote but won the presidency. This includes former President Donald Trump, who lost the popular vote to Democrat Hillary Clinton in 2016 by nearly 2.9 million but still won enough votes in the Electoral College to become president.

This often sounds crazy to people who live in democracies in the rest of the world. The U.S. is the only country to have a system where voters select a body of electors with the sole function of choosing the president. In most other democracies, the president is directly elected through the popular will of the voters.

Each state's presidential electors are equal to the number of its representatives in the U.S. House and Senate. This benefits smaller states and sets the stage for presidential elections to largely hinge on just a handful of swing states.

A presidential candidate must win a majority of the 538 total electoral votes to win (the District of Columbia gets three). Most states use a winner-take-all system in which all electors award their votes to the popular winner in the state. Maine and Nebraska are the exceptions, awarding theirs on a proportional basis.

The Electoral College incentivizes presidential campaigns to focus visits and spending on a small number of swing states.

This year’s presidential battleground states represent 18% of the country’s population but have dominated the attention of the Democratic and Republican presidential candidates and their running mates.

Through Tuesday, the Democratic and Republican tickets have had just over 200 total campaign stops — three-quarters of which have been to the seven battleground states, according to a database of campaign events that is based on Associated Press reporting. Pennsylvania alone has been visited 41 times, the most of any state. The AP data shows Michigan is second, with 31 visits through Tuesday, followed closely by Wisconsin, with 27. The rest: North Carolina, 18; Nevada, 13; and Arizona and Georgia with 12 visits each.

But it’s not just the state visits: The presidential campaigns are tailoring their appearances to specific counties they believe are crucial to their success. The AP’s database shows their campaign events in those seven states have been concentrated in counties with 22.7 million registered voters — just 10% of all voters registered nationally for this year’s presidential election.