Why did the republicans lose seats in the 1972 senate elections?

[Del Tachi]

Because Nixon largely just tried to get himself re-elected at the expense of everything else...everything...else.

Only Hillary's 2016 strategy comes even close to being so neglectful.

Nixon notably refused to campaign for a lot of Southern Republicans in 1972.  It was probably a better strategy for him overall though, as incumbent Southern Democrats weren't going to lose and Nixon had relied on their votes previously.