Going below 30 per cent kept Truman from seeking another term and going below 30 per cent eventually drove Nixon out of office.
In the modern era, beginning with FDR, Presidents get into trouble when they fall below 30.
President Nixon resigned after his approval rating fell below 30 per cent (Picture: CBS)
No president has been so unpopular so early in his first term
Looking at the Monmouth University poll released last Wednesday, the largest drop in Mr Trump's approval was among women.
Just 24 per cent of women approve the former businessman's job performance as President.
"This result is not good for the president, especially coming off the loss of his endorsed candidate in the Alabama Senate race.
"Republicans have to be worried about being dragged down by the weight of Trump's negatives in 2018 if this trend continues," said Patrick Murray, director of the independent Monmouth University Polling Institute.
But Republican strategist John McLaughlin has argued that Trump isn't in as bad a position as the polls make out.
He does agree however that what happened in Alabama could be a catastrophe for the Republican party.
A Republican stronghold, the Democrats won Alabama this week after Roy Moore became engrossed in sexual misconduct allegations.
Mr Trump spent a lot of political capital on the special election, essentially getting nothing out of it.