Question

During the presidential election, the candidates campaigned heavily in states like Virginia, Pennsylvania and Ohio. Why?

Answer 1

These states are most likely to be easily swayed either way. Heavy campaigning would increase the chances of that state having an electoral vote for that candidate.

Answer 2

They are "swing" states, as wombat points out, and also they each have a substantial number of electors in the Electoral College, so winning them is very worthwhile.