Why did the popular election of presidents mean that campaigns now had to be conducted on a national scale?

All voters should be valued equally in presidential elections, no matter where they live. Our current Electoral College system, grounded in state laws which allocate electoral votes on a winner-take-all basis, leads presidential candidates to concentrate their resources on voters in a handful of swing states, relegating the vast majority of the country to spectator status. Instead, we should elect the president by a national popular vote—and there's a state-based, constitutional way to do so: The National Popular Vote interstate compact.

Explanation:

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one of the main reasons was definitely slavery. back in 1861, republican abraham lincoln was elected and ended up being a major influence in the slavery business. slavery greatly affected the texas economy because back in 1845 during the annexation of texas, slaves became a big part of the texas population. many texans that were in favor of the secession influenced groups through party ties.