Former U.S. Secretary of Defense Donald Rumsfeld said Wednesday that he will support Donald Trump with his vote in November.

The declaration, which Rumsfeld made in an interview with DailyMail.com, makes him the highest-ranking member of the George W. Bush administration to back the Republican Party's presumptive nominee.

It's 'not a close call,' he said in a 25-minute phone interview.

Rumsfeld said he agrees in principle with Trump's position on reforming the NATO alliance, keeping Syrian refugees at bay over fears of terrorist infiltration, and other issues.

And besides, he added, 'I'm a Republican, and there's not any doubt in my mind how I'll vote,' and 'I don't believe Hillary Clinton is qualified to be President of the United States.'

KNOWN UNKNOWN: Donald Rumsfeld tells DailyMail.com that he will be voting for Donald Trump in November because the questions marks about his potential presidency are far more acceptable than what Americans already know about Hillary Clinton

'I am incredibly grateful for the support of Secretary Rumsfeld, and I am very honored he supports my stances on NATO, the need to defeat ISIS and stop immigration of Syrian refugees,' Trump told DailyMail.com in a statement.

Bush, the 43rd president, has given no indication that he will back Trump, who bounced his younger brother Jeb from a vicious GOP primary process this year.

The former president has indicated that he won't attend July's Republican National Convention in Cleveland, an event already being talked about as 'The Trump Show' inside party circles.

Colin Powell, the Bush-era secretary of state, has been openly hostile to Trump over his immigration positions.

'Let's tell all the immigrants working in Trump hotels to stay home tomorrow. See what happens,' he told the Washington Ideas Forum last October.

Rumsfeld said Powell's opposition to his party's standard bearer is unsurprising.

'He supported Obama, Colin did! That's nothing new,' he said.

Rumsfeld spoke with DailyMail.com as part of a media tour aimed at promoting the release of his 'Churchill Solitaire' app on the Android platform.

The iPhone version has been downloaded more than 650,000 times and represents Winston Churchill's favorite version of the play-alone card game.

Rumsfeld said he learned the game in 1973 from André de Staercke, a Belgian NATO diplomat, who learned it 30 years earlier from Churchill himself.

'I was afraid it would be lost to the ages,' he said Wednesday, while admitting the technology mystified him at first.

'I didn't even know what an app was!' he marveled. 'Don't tell anyone.'

TOUGH TALKER: Donald Rumsfeld is seen in Kuwait in June 2002, shortly after the fall of Saddam Hussein

CHURCHILL SOLITAIRE: Rumsfeld is promoting the Android version of his popular app fashioned after Winston Churchill's favorite play-alone card game

An animated Rumsfeld, 83, was even more eager to talk about the Trump phenomenon, saying that a year ago 'you could count on one hand' the number of people who thought he would be the GOP nominee.

While the former defense secretary said he and Trump have never met, he agrees with the real estate tycoon about what Trump calls the potential for a 'Trojan horse' infiltration of terrorists among the Syrian refugees whom the Obama administration has been resettling in the U.S.

'He's absolutely right,' he said. 'Anyone who thinks the radical Islamists are not going to try to utilize every venue they can find to infiltrate in the United States, and in western European countries, to achieve their goals – these people just don't get it.'

'They've announced what they're going to do,' he said of the ISIS terror army.

On the refugee crisis and the need for a NATO reboot, Rumsfeld said he has arrived at a Trump-aligned position through a different line of argument.

'We've got an enormous obligation' to help fleeing Syrians, he said, 'and the United Nations is so incompetent and inept and unhelpful ... this refugee problem is the most heartbreaking thing in the world.'

But instead of politicizing the issue and moving the refugees halfway around the world, he said, the U.S. should be mobilizing regional powers to give them somewhere to flee to.

'We've got an obligation to provide safe zones for them,' he said. 'We've got to be proactive rather than arguing whether we should have 1000 or 2000 or 3000' in the United States.

'How is what I've said different from what he's said?' he asked, referring to Trump – whose position is nearly identical but contains an extra layer of terrorism rhetoric.

'The way he said it got him the Republican nomination for president. The way I said it makes people's eyes glaze over' but is 'less concerning to some people.'

'The only real difference is [that] one is striking a nerve.'

HE'S NOT WITH HER: Rumsfeld said Hillary Clinton isn't qualified to be President of the United States

Trump's immigration policy and his own views, he said, similarly arrive at the same end but form different starting points.

Most immigrants, he said, don't 'want to impose a different system on us.' But 'countries ought to be able to maintain their values' in the face of immigrant populations that don't want to assimilate.

On NATO, Rumsfeld emphasized that most media reports of Trump's stated goals have been inaccurate.

'People have played very fast and loose,' he said. 'What he actually said was different from the characterizations that followed.'

Rumsfeld, once America's NATO ambassador himself, agreed with Trump that the internal compass of the organization needs an overhaul, proclaiming the presumptive nominee's position is 'not far from what I wrote ... to President Bush 13 or 14 years ago.'

NATO, he said, was 'fashioned during the Truman administration, and they do need to be updated for the 21st century and the information age.'

Hillary Clinton has pounced on popularized but inaccurate versions of Trump's NATO pronouncements, saying he would withdraw the U.S. from the alliance entirely.

Rumsfeld framed the choice between Clinton and Trump in terms political historians will find familiar, relying on the words he used in 2002 to describe questions about the U.S. intelligence community's ability to spot weapons of mass destruction in Iraq.

'Mrs. Clinton is a known known. Donald Trump is a known unknown who's a recent entry into the equation,' he said, attributing the insight to his wife.

'And I am a lot more comfortable with a known unknown, who I will support, than with a known known who is unacceptable.'

'I couldn't support Mrs. Clinton,' he emphasized. 'What she has done with classified information I think is inexcusable and puts at risk people's lives [and] not just ours.'

Clinton is facing an FBI investigation concerning classified material found among emails she housed on a homebrew private server while she ran the State Department.

'I personally believe that if she were a sergeant in the Army or a yeoman in the Navy, she'd probably be indicted,' he said. 'It's just inexcusable. You just can't do that!'

The impact of Clinton's data security practices will be far-reaching, Rumsfeld predicted.

'Other countries will be less likely to share information with us,' he said, now that it's clear the U.S. government can't maintain the secrecy it once boasted as a core competency.

And the potential for hacking, he said, means state secrets are falling 'into the hands of the other side,' which 'tells them that we had the information, which means they can figure out how we got it and stop us from getting more ... and even compromise the people who gave it to us or allow us to get it.'

Rumsfeld also took issue with Clinton's statements to family members of U.S. personnel killed in a 2012 terror attack on a diplomatic post in Benghazi, Libya.

Those next of kin have said Clinton blamed their loved ones' deaths on an anti-Islam YouTube video, rather than identifying the carnage as the result of a jihadi attack.

Her emails at the time show that she conceded the truth to her daughter and to foreign leaders.

'When Mrs. Clinton said to those people, the parents of the people killed, that it was the video, when she knew completely, certainly, that it wasn't' – he said, suddenly falling silent.