There is an argument that the less moderate candidate has won every presidential election since 1980.
A few things to keep in mind:
- The idea that moderates do better than extremists is not some sort of law.
Statistically it appears to be the case, but there are exceptions. Sometimes external factors will come into play that will harm a candidate's chances regardless of how moderate/extreme they are. Or sometimes the candidate will just run a bad campaign.
- When deciding whether a candidate is moderate or extreme, you have to look at most/all of their policies, rather than just focusing on one element and labeling them moderate/extreme based on that. You also have to consider their position in relation to ALL the other candidates and party members. Just because a republican candidate doesn't hold the same positions as a democrat does not make him 'extreme'.
- The claim that we are making is that a moderate has a better chance than an extremist. If both candidates are moderate or both extreme, then the argument becomes irrelevant.
- There are certain aspects of the American political system that put Democrats at a disadvantage. (Electoral college, voter suppression). That means that even if being a moderate is an advantage, it could mean that they still lose because of those other disadvantages.
So lets look at your list....
Reagan first came to power after Jimmy Carter (a.k.a. "History's greatest Monster"). Carter had to deal with the Iranian hostage crisis and the invasion of Afghanistan, both factors that would have harmed any candidate.
While the presidency of Bush Sr. was often seen as a continuation of Reagan's term, Bush himself was actually more moderate. Remember, he at one point labeled Reagan's plans 'voodoo economics'. And as president he supported stronger environmental protections and increased rights for people with disabilities.
So, I'd say overall Bush Sr. was a moderate, even before he was elected.
Clinton (Bush raised taxes and reverted to being a moderate)
Clinton (This is at best unclear... Clinton was GOP light at this point but so was Dole, really)
I'd say Clinton was a moderate in both of his campaigns.
Bush ( how could you out moderate Gore?)
Bush (ADAM AND EVE NOT ADAM AND STEVE; SUPPORT OUR TROOPS OR THE BABY JESUS WILL CRY)
Yes, Bush 'won' against Gore. But he lost the popular vote. Now, you could argue that 'a loss is still a loss', but the problems of the electoral college will impact ANYONE the democrats select as a candidate.
Obama (McCain's whole trip was reasonableness, except his VP choice was maybe so bad as to flip this one)
Obama (Romney is like mayo, only without all the spiciness)
Obama was pretty much a moderate as a president. His 'health care reform' was an improvement over what existed before, but wasn't anywhere near as radical as "medicare for all". He brought in new financial regulations, but he didn't go as far as breaking up the banks. He was generally on the left when it came to social issues (abortion and gay rights), but his actions on gun rights were limited. (He wanted more regulations, but certainly didn't call for an outright ban.)
Again, another case where a candidate lost the popular vote, but still won thanks to the electoral college (not to mention voter suppression and Russian collusion.)
I'm not sure trying to drift back to the center is a winning move.
Well, given the fact that some of the 'extremists' you listed were actually closer to being 'moderates', and that some of the 'extremist' victories were questionable at best, I'd say seeking out the political center is a good idea.
Could Sanders have beaten Trump in 2016? I do not know. Clinton had problems with her campaign that had nothing to do with her position on the political spectrum that Sanders would have avoided. But, the issues of voter suppression, Russian interference and the electoral college were still there, and he would have been impacted by them too.
And I am not discounting Sander's chances should he become the nominee in 2020. Its possible that Trump's problems will finally catch up to him. Its possible that his personality will be convincing enough to people. Its possible that republican actions (the tax plan, attempts to kill Obamacare, etc.) will be seen as a hard-shift to the right, so that it will be a case of far-left vs. far-right.
I'm just saying that
statistically there are reasons that moderates do better
on average than extremists.