So former U.S. Rep Charles Djou told his fellow Republicans not to support Donald Trump — the day before caucus-goers soundly chose Trump as their presidential favorite.

“Mr. Trump has built his presidential campaign around ugliness and anger,” Djou wrote Monday in an open letter to Island Republicans.

Way too little, way too late.

Five years ago, when Trump’s “birtherism” was picking up steam, Hawaii Republican leadership — including Djou — had a real opportunity to deal Trump a serious blow. From then-Gov. Linda Lingle on down, they believed birtherism was silly at best, racist at worst.

But they passed.

In April 2011, I emailed the senior State House Republican, Rep. Gene Ward, this proposal:

“Nothing would have more credibility and clout in debunking the bogus birther claims than a forcefully worded, nationally released press statement from Hawaii’s Republican leadership.

“Now, with Trump’s poisonous embrace of birtherism, it’s gained more traction and degrades the institution of the presidency.

“Republican leadership in Hawaii has a responsibility to publicly denounce this Birther nonsense. The only reason I can think of that you would keep turning a blind eye is that you secretly hope it will damage Obama’s 2012 re-election prospects.”

Ward didn’t admit — but also didn’t deny — that inaction served the party’s national political interests.

Although he said he believed that Obama was born in Hawaii — because he had “talked to a person” who had seen Obama’s long-form birth certificate — Ward refused to propose to his party peers that they make a forceful group statement aimed at a national audience.

A “profile in courage” this wasn’t.

Slom: Birther Concerns ‘A Legitimate Issue’

But the worst of the lot was state Sen. Sam Slom.

Virtually all local Republicans, when asked, would dismiss the birther allegations. But Slom flirted outright with them.

He said that while he believed Obama was born in Hawaii, questions about what was in Obama’s (not yet released) long-form birth certificate were “a legitimate issue.” (This despite the 2008 release of Obama’s short-form “Certification of Live Birth,” the official document used by Hawaii-born residents to prove their citizenship and obtain a passport.)

Slom told a New York City radio station that there must be something in his birth certificate he wanted to hide — the assumption being it threatened his legitimacy as president.

Today, Slom supports U.S. Sen. Ted Cruz for president. Like Trump, Slom must want to put the birther business behind him.

But it won’t go away.

When something like Trump’s birther idiocy is ignored, it spreads like a case of bad mold.

In May 2011, shortly after Obama released his long-form birth certificate, a Gallup poll found that 23 percent of Republicans still doubted he was eligible to be president.

In January 2016, a HuffPost/YouGov poll found that 53 percent of Republicans do not think Obama was born in the United States, more than double what it was when Slom went birther five years ago.

For Ward, Djou and the other Hawaii Republican leaders who chose public silence in the face of Trump’s birtherism when it was first gaining traction, shame on them.

As for Slom …

This year he may face a formidable challenge for his state Senate seat from former City Councilman Stanley Chang.

Slom has had a rough year — health issues and a lawsuit filed against him by his former live-in girlfriend (of 14 years), journalist Malia Zimmerman.

Today, Slom’s health is on the mend. The Zimmerman litigation was settled in November.

If he loses his Senate seat, though, Slom will have plenty of free time to contemplate his small role in pumping up Trump and his birther posse.