Why is Ohio so important in Presidential race?

Ohio is so important in the election season because it is a swing state. This means it is one of the few states in the Union that is not heavily Democratic or Republican and can vote either way. These states are often what decides the outcome of the election. Florida and Nevada are other swing states.

Ohio has been a swing state in most Presidential elections, in large part because its population mirrors that of the US as a whole. No Republican candidate has ever won the presidency without winning Ohio's 18 electoral votes, the most of any swing state except Florida (27 votes), which has only really been competitive since the extremely close 2000 election.