Why are the Bushes declining to endorse Donald Trump?

We don't yet know why the last two Republican presidents indicated Wednesday that they will not endorse their party's presumptive 2016 nominee, Donald Trump.

The statements George W. Bush and George H.W. Bush released through spokespeople provided no hint of a motive. 43's spokesman, Freddy Ford, said simply, "President Bush does not plan to participate in or comment on the presidential campaign."

Jim McGrath, 41's spokesman, provided a slightly lengthier - and slightly less credible - explanation: "At age 91, President Bush is retired from politics. He naturally did a few things to help Jeb, but those were the 'exceptions that proved the rule.'"

Of course, George H.W. Bush wasn't too far removed from politics to wade into the presidential race four years ago, when he "enthusiastically" endorsed his "old friend Mitt Romney."

So why are they sitting this one out? It's safe to say the Bush family has little affection for Trump, given the way the presumptive GOP nominee has portrayed them -- particularly George W. and Jeb Bush -- throughout his campaign.

He relentlessly ridiculed Jeb Bush, perhaps more than any other candidate - for his policy proposals, for his flagging poll numbers, for his "low energy" demeanor.

But their ugliest spat came during a discussion of 9/11 and Iraq. During the CBS News Republican debate in February, Trump was asked whether he stills believes, as he once said, that it would have been a "wonderful thing" for Congressional Democrats to impeach former President George W. Bush over the war in Iraq.

"Obviously, the war in Iraq was a big, fat mistake," Trump said. "George Bush made a mistake. We can make mistakes. But that one was a beauty. We should have never been in Iraq. We have destabilized the Middle East."

"So you still think he should be impeached?" asked "Face the Nation" host John Dickerson.

"You do whatever you want," Bush said. "You call it whatever you want. I want to tell you. They lied. They said there were weapons of mass destruction, there were none. And they knew there were none."

Jeb Bush was visibly agitated, grumbling in response, "I am sick and tired of him going after my family. My dad is the greatest man alive in my mind, and while Donald Trump was building a reality TV show, my brother was building a security apparatus to keep us safe."

Trump was quick with a retort: "The World Trade Center came down during your brother's reign, remember that."

In an interview with "Face the Nation" the day after the debate, Trump clarified that he was not "blaming" George W. Bush for 9/11, but he added, "The CIA said there was a lot of information that something like that was going to happen...when Jeb gets up and says, we were safe under his brother, we weren't safe."

And Trump didn't let up on the subject of Iraq, either, calling the war a "disaster" that may lead to "the destruction of Europe" because of migrants fleeing the Middle East.

"We would have been so much better off if Bush and the rest of them went to the beach and didn't do anything," he said. "You had Saddam Hussein - he was a bad guy and all of that, but he made a living off killing terrorists. Now if you want to become a terrorist you go to Iraq. That's like the Harvard of terrorism."

It was a startling exchange: the man who would eventually win the 2016 GOP primary, utterly repudiating the defining act of the last Republican to occupy the Oval Office. Trump didn't win the Republican primary while ignoring the Bush family's legacy - he won while gleefully stomping all over it.

Ultimately, until we get more than a pair of terse statements from the former Presidents Bush, any attempt to explain their decision to sit on the sidelines during the 2016 general election boils down to an educated guess. But you don't have to look too far to find one.