Trump is one of only two presidents to increase his vote share from a winning run to a losing one

[The Mikado]

Donald Trump, 2016: 45.9% of the vote, wins.
Donald Trump, 2020: 46.8% of the vote, loses.

Closest thing I could find at first was Grover Cleveland winning with 48.8% and then losing with 48.6% (and then winning again with 46.0%) but it's still a decline even if it is an insignificant one.

Then...I found JQA. Of course 1824 was a weird, weird election, so I think I'm gonna say JQA doesn't count, but if we do count him, JQA goes from 30.9% and winning to 43.7% and losing.

[morgankingsley]

So if we dont count JQA then trump be the only?

[Skill and Chance]

So if we dont count JQA then trump be the only?

Depends on how you handle Cleveland.  His vote share was 1884 (win) >1888 (loss) >1892 (win).  I think winning back the WH in 1892 with less of the vote than he got in his 1888 loss should qualify him for this?

[Chips]

Interesting data.

[RI]

On a similar note, the only Presidents to increase their total vote upon running for re-election and still lose are:

John Quincy Adams (received more votes in 1824 than were cast for all candidates in 1820)
Martin Van Buren
Grover Cleveland
Donald Trump

Harrison, Taft, Hoover, Carter, and Bush all received fewer votes in their losing re-election campaigns.

[Orser67]

Makes sense considering that:

a)we've only had 10 presidents lose re-election in the general election (not counting Ford), and
b)all else being equal, you would generally think that a candidate who won the first time would probably win the second time if they increased their share of the popular vote.

[Skill and Chance]

This also suggests the high 3rd party vote was the biggest factor in Trump's win. 

[Statilius the Epicurean]

This also suggests the high 3rd party vote was the biggest factor in Trump's win. 

Yes, it suggests how much of a fluke Trump's victory in 2016 was. There was always an anti-Trump majority that only had to coalesce to beat him.

[AGA]

Well, not super surprising. Despite a two-point D swing, Trump still got a higher vote share just due to the decreased third-party vote.