The last two Republican presidents will not endorse the Republican nominee, Donald Trump, in the general election, and spokesmen for George H.W. Bush and George H. Bush say neither man will comment on the contest.

Fox News:

George W. Bush's personal aide, Freddy Ford, said that his boss "does not plan to participate in or comment on the presidential campaign."

"At age 91, President Bush is retired from politics," Bush 41 spokesman Jim McGrath wrote in an email to the website. "He came out of retirement to do a few things for Jeb, but those were the exceptions that proved the rule."

According to the Tribune, the elder Bush has endorsed every GOP presidential nominee since losing his 1992 re-election bid to Bill Clinton. George W. Bush also campaigned on behalf of Sen. John McCain in 2008 and Mitt Romney in 2012. 

Both Bushes campaigned heavily for Jeb Bush earlier this year, but he dropped out after disappointing results in the first three presidential contests. Neither former president made an endorsement during the rest of the primary season, though George W. Bush was recorded last year telling donors "I just don't like" Sen. Ted Cruz. 

The younger Bush has also taken veiled jabs at Trump, telling a South Carolina audience in February that "we do not need someone in the Oval Office who mirrors and inflames our anger and frustration."

"Strength is not empty rhetoric," Bush also said at the time. "It is not bluster. It is not theatrics. Real strength, strength of purpose, comes from integrity and character. And, in my experience, the strongest person usually isn't the loudest one in the room."

Trump is no fan of the Bushes, having called Bush 43 a liar about the claim that there were WMD in Iraq before the invasion.  But the decision by two former presidents not to endorse their party standard-bearer is a symptom of Trump's biggest challenge that he must overcome to win the White House.

There is no doubt that Donald Trump will attract a lot of Democrats in November.  Some of his supporters claim that this alone will make him competitive with Hillary Clinton.

Mitt Romney was able to attract 93% of the Republican vote in 2012.  What percentage of Republicans will support Trump?

Right now, it doesn't look good for The Donald.  A recent Suffolk University poll shows that fully 40% of Republicans would not vote for the nominee if it is Trump, and 19% of those would support Hillary Clinton.  Nearly half of Republican women would refuse to support him.

It's a given that these numbers will shrink the closer we get to the election.  But Trump can't count on an increase in turnout of white males to offset his loss of support among Republicans.  That's because there just aren't enough of them.

The bottom line: Trump has to win almost as many Republicans as Romney did in order to have a chance.  Even a fall-off of 5% would mean curtains for The Donald. 

That makes unifying the party a lot more important than Trump is letting on.