This is more along the lines I was thinking of.
I was not thinking of the standard careful, reserved, mealy-mouth stuff that Ex-POTUS usually engage in. Dubya has already done some of that.
It’s gonna be a close election. I can’t believe that Dubya couldn’t move 100,000 or so swing voters.
Which swing voters are those? The ones who didn’t like Bush’s handling of the Iraq war, the economy, or Katrina?
Let me put it differently. All the crap that’s come out about Trump over the past three years – all the golf weekends, pushing business to his hotels, paying off a porn actress, muscling the Ukraine, and all the other stuff that’s too much to go into now – all of that hasn’t budged his approval numbers a bit. What do you expect Dubya to accomplish?
If Dubya launched an all out assault on Trump, it probably won’t change the outcome of the election, and it might cause him to accidentally fly out of a window somewhere…
It certainly made the major media sources… so quite possibly not many. 
It didn’t seem to resonate with the more engaged to reach everyone else.
McCain’s opposition was pretty regular, even after the election, until his illness pulled him out of DC. That seems like it would be harder to miss but ignorance is vast.
None of them would have had the kind of campaign or personal funds to make major national media buys to really move the needle on awareness. The Clinton campaign and major Democratic PACs would have had the funds and didn’t pick up the ball. Bloomberg could have pushed it from petty cash. The statements were out there already. Since Trump’s actual opponents didn’t run with it, I would guess they didn’t see any value in the attack.
How about a thought experiment. Imagine a magical being appears to you and says that at the stroke of midnight on Halloween 2020 you will suddenly be able to control G.W. Bush’s mind and actions to any extent you care to until November 4, 2020.
Seeing an opportunity, you decide to use this ability to destroy Trymp’s chance at re-election. But you gotta be smart… gotta plan this out… what to do, what to do? You could make some sort of public comment. Should it be clear and obvious? Or maybe subtle insinuation? Imagine that power! There are all sorts of fun things you could puppeteer Dubya into doing for fun and profit, but what about doing something for the greater good?
I think you guys are underestimating the amount of political and social capital an ex-president has. Theses guys can still make a lot of things happen, just now it’s in the private sector. Mostly.
Ex-POTUSs are part of American Royalty along with the current prez is, his/her family, and Oprah.
Maybe he could spin a huge tale of CIA, FBI, LSD, Ukraine, bribery, car-chase intrigue. He wouldn’t sound like a loon if he said the right things. It’d be taken seriously coming from a GOP prez.
If not that, got a better idea? What serious things could he say near the election that would siphon off at least 100 votes? And if 100, what about 1000? Etc.
And just like British royalty, we like them because they don’t get involved in politics.
No, I don’t have a better idea because I think your assessment that George W. Bush can say or do ANYTHING to affect the re-election of Donald Trump is wrong. Look at it this way. Bill Clinton is a lot more popular than GWB. What do you think he could say that would make swing voters who were leaning toward Bernie suddenly switch their votes to Trump?
That’s probably the most telling thing; they’re going to tend to view W as being half-assed in his policies relative to Trump, and regard anything he says as being the product of the deep state and establishment, not of a (presumably) older, wiser, more experienced political elder.
The single biggest thing working against anything like this is the cultish following that has developed around Trump- it’s not just Republican vs. Democrat or Conservative vs. Liberal any more- it seems more personal for a lot of the Trump supporters. He’s THEIR guy, not the party’s guy. You see it around here (Texas) in the Republican primary ads- pointing out who’s in cahoots with Trump is apparently a very positive thing according to the ads Somehow they’ve managed to wind Trump up with some sort of right-wing, evangelical/conservative Christian imagery- like he’s been anointed by God to carry the torch for their particular views on things.
It explains why the rest of the GOP is so feckless and is sticking with him- his cultish followers are as likely to turn on them for lack of support as they are to defend vs. the Democrats. So they stick to him for lack of a better alternative I suspect- they either stick with him, or get denounced as a RINO and abandoned at the polls for someone who will stick with him.
Large swaths of the GOP electorate these days view George W. Bush as an establishment has-been. If you mention his name to many Republicans these days, even those who loved him and voted for him twice back in the 2000s, you’ll likely be met with a chorus of “George who?”
And I seem to recall he campaigned pretty hard for Jeb! in South Carolina back in 2016. Jeb! still finished in fourth place.
His supporters would scream fake news and say that George W. is just jealous of Trump’s awesomeness.