2018 election: Why do we have midterm elections between presidential election years?

In the United States, we head to the polls every four years to elect a new president. So why do we have midterm elections halfway through each presidential term instead of just voting for everything at once every four years?

The answer has to do with term lengths established in the Constitution that vary between different elected offices.

In the U.S. House of Representatives, members are elected to two-year terms. All seats in that body are on the ballot during every midterm and presidential election year.

In the U.S. Senate, meanwhile, members are elected to six-year terms. Their terms are staggered, and a third of the seats are up for grabs every two years. Click here to see which senators are up for re-election in 2018.

State and local elections for ballot initiatives and elected offices like governor, mayor, county judge or county executive, judge, sheriff and others can take place in any year, according to USA.gov. All but two states elect their governors to four-year terms, and more than half of governor elections fall on midterm years. Click here to see which governors are up for election in 2018.