1.61 votes for Nixon for every vote for McGovern, the real reason Democrats hated him so much.
The smallest popular mandate belonged to JFK over Nixon in 1960 at 1.0033. Democrats stole that election but Nixon let it pass for the good of the country.
There were smaller mandates, if you count Bush 2000 at 0.98 (Gore 1.01), or Trump 2016 at 0.95 (Hillary 1.04), lol, but those aren't really popular vote mandates now are they?
LBJ was second in 1964 with 1.58, because JFK had been assassinated in 1963. I don't think Jesus himself could have won it for the GOP in 1964.
Third overall was Reagan in 1984 with 1.44, followed by IKE at fourth and fifth with 1.36 and 1.24 respectively in 1956 and 1952.
If back-to-back terms is your yardstick, Reagan was tops with a combined mandate of 1.335, followed by Nixon at 1.31, IKE at 1.30, Bill Clinton at 1.17, and Obama at 1.115. Bush 43 brings up the rear at a distant 1.015.
People who think Trump is a repudiation of Bush 43 Republicanism should consider that Trump's not-back-to-back combined score is now 0.99.
I've said it before and I'll say it again: Trump is a transitional figure for the GOP, and certainly not a nationally transformational one like Reagan who destroyed the bipolar world, IKE who re-moralized a victorious but demoralized nation, and Nixon who fatefully opened the door to China.
Unfortunately for the GOP, Trump is mostly just a wrecking ball who is wrecking his own HDQ, and the Democrat threat remains just decimal points away.