What are some data points, which may help put the chance, of the Democrats winning against Donald Trump in this election?
How often does an insurgent win against an incumbent?
What other statistics related to this might be of use when trying to estimate the general odds of this election?
Wikipedia has a list of the presidents who failed to win reelection. This includes ten United States presidents.
From the list of US presidents, we can find twenty-two presidents who won re-election (counting Franklin Delano Roosevelt three times).
Total, that's thirty-two attempts with twenty-two successes. That's 68.75%.
The Washington Examiner did a study starting from 1900 through 2004. There were fourteen successes and five failures. That's 73.68%. But since then, Barack Obama won a fifteenth time. That's 75%.
These include presidents like Teddy Roosevelt and Calvin Coolidge who won election as an incumbent after replacing a previous president. And this does not include Lyndon B. Johnson in 1968, when he dropped out (anticipating a loss if he ran).
Past performance does not guarantee future results. But the basic chance is around two-thirds to three-quarters. There are models of these things. However, there aren't really enough data points to make for good models.
There are several models of this. Here's an overview by the Brookings Institution. Warning, they have a distinct liberal bias in general, so read with skepticism.
One problem is that these models tend to use information that they don't have yet. For example, from that overview:
For example, Alan Abramowitz’s “Time for Change” forecasting model, based on the incumbent president’s net approval rating at midyear in the Gallup Poll, the growth rate of real GDP in the second quarter of the election year, and whether the incumbent president’s party has held the White House for more than one term, produced the most accurate prediction of the 2012 presidential election among this set of forecasting models.
So we don't yet know two of the three things that the model uses. We could guess, but then the model would be subject not just to its error but to the error in the guesses.
This is highly dependent upon the state of the economy.
The last three presidents to fail in a re-election bid were Bush 1, Carter, and Hoover.
In all three cases, the nation's economy had taken a sharp downturn since they had won their first term. There were other factors at play, but the only major element of commonality in all of those losses was a falling economy.
Based on those events, much of Trump's chances of re-election rest on the state of the economy between now and fall 2020, data points that don't exist at this time.
I’m not aware of any generic attempt to measure this. However with respect to US presidential elections, you may be interested to look at the “Keys to the White House” model. There is a list of thirteen variables which are said to predict whether the incumbent will win or lose. By popular vote, the outcome has been consistent with this model in every US presidential election since 1860.
In United States history, incumbents have run for re-election 32 times, and in 22 cases (68.75%) they won re-election. Four of those victorious incumbents were Vice Presidents who had assumed the Presidency upon the death of the President (T. Roosevelt, Coolidge, Truman, L. Johnson), so another way of looking at it is that only 17 out of 44 Presidents (38.63%) were elected more than once. (F. Roosevelt was re-elected three times. Grover Cleveland was elected twice, but not consecutively.)
Neither of these statistics really helps us to predict what will happen in 2020. If we want to look at only recent history, almost every President in living memory has been re-elected to a second term, with notable exceptions of Gerald Ford, Jimmy Carter, and George H. W. Bush.
Ford was never elected President, or even Vice President, but succeeded to the Presidency after both President Nixon and Vice President Agnew had resigned, so I'd say he was a pretty weak candidate. In the other two cases, 1980 and 1992, there were third party candidates who captured an unusually large portion of the vote. Whether that was a cause, or an effect, of the weak performance of the incumbent, is a question for historians to debate.
Prior to Ford, the last President who lost his re-election bid was Hoover in 1932.
