In 1988, Republican presidential nominee (and incumbent vice president) George H.W. Bush won 40 out of 50 states and a 7.8% margin in the popular vote. On the same day, Democrats in the House of Representatives won an 85-seat majority with a 7.7% lead in the popular vote. That's a 15.5% discrepancy between the popular votes for President and House.
In 1984, Republican Ronald Reagan won re-election carrying 49 states and he won the popular vote by 18.2% while Democrats won a 71-seat majority in the House and won the popular vote by 5.1%. That's a 23.3% discrepancy.
In 1980, Reagan was elected with 44states and a 9.7% popular vote margin. Meanwhile in the House of Representatives, Democrats won a 51-seat majority with a 2.6% lead in the popular vote. That's a 12.3% discrepancy in the popular vote.
Going back further to 1972, Republican president Richard Nixon was re-elected with 49 states and a 23.2% margin in the popular vote. At the same time House Democrats in the house won a 50-seat majority with a 5.6% popular vote advantage. That's a 28.8% discrepancy.
Why were there such a huge discrepancies? This has not happened since 1988.
In 2016, the discrepancy between the popular votes for president and house was only 3.2%. In 2012, it was 2.7%. In 2008, it was 4.4%. In 2004, it was 0.2%. In 2000, it was 1%. In 1996, it was 8.5%. In 1992, it was 0.6%.
Nowadays it seems most voters give all of their votes to candidates from the same party (at least for federal elections). So it's hard for me to understand how 30-40 years ago large numbers of voters were splitting their votes. What changed? Why were voters so willing to split votes 30+ years ago but not today?