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One of the nation’s most prominent social conservative leaders, an Iowa Republican whose endorsement GOP presidential candidates aggressively pursue, says that one key reason he opted not to throw his support behind Sen. Marco Rubio was because his youthful appearance works against the public seeing him as presidential.

In an exclusive interview with Fox News Latino, Bob Vander Plaats – whose endorsement in 2008 of former Arkansas Gov. Mike Huckabee and in 2012 of Sen. Rick Santorum was seen as crucial to their victories in the Iowa caucus – said he had given serious consideration to endorsing either Rubio or Sen. Ted Cruz before deciding on the Texas firebrand.

“It wasn’t an easy choice,” Vander Plaats said in a recent interview conducted in a conference room of the spacious offices of his organization, The Family Leader. “There’s a lot I like about Marco. It was coming down to two people – not Trump and Cruz, but Cruz and Rubio.”

He called Cruz “a man of high character, he’s a principled conservative.”

People have to be able to see you as president...The visual of ‘I can’t see him as being president’ is important.

— Bob Vander Plaats

“Ted is very bright. He’s got a record of accomplishment – his (Texas) Attorney General record, his arguments before the U.S. Supreme Court. He led the way he said he would in the Senate. He took on Democrats and Republicans, in a respectful way.”

Vander Plaats said he consulted with other conservatives he holds in high esteem, and found that they could not envision Rubio, 44, as president.

“Many said to me that they like him a lot, but they see him as vice president,” Vander Plaats said. “To his credit, he looks 19. I think he gets carded if he tries to buy a beer.”

It may seem unfair, and even arbitrary, for youthfulness to work against a candidate in the public’s mind, he suggested, but optics cannot be ignored.

“People have to be able to see you as president,” Vander Plaats said. “The visual of ‘I can’t see him as being president’ is important.”

Rubio's campaign said it would not comment on Plaats' remarks.

Though he stressed that he is firmly for Cruz, Vander Plaats added that Rubio could still seem presidential by being more authoritative and clearer on immigration.

“He’s got plenty of chance for recovery. “

Ted Cruz, incidentally, is only six months older than Rubio; Cruz turned 45 in late December. Rubio will turn 45 in May. But, unlike Rubio, Cruz has not faced criticism about his youth or about running for president after being only a first-term senator. Political experts say it comes down, once again, to optics.

“Most of the Republican presidential candidates, from Ronald Reagan and George H.W. Bush to John McCain and Mitt Romney have generally been people in their 50s and 60s or older,” Cal Jillson, political science professor at Southern Methodist University in Dallas, told Fox News Latino in November. “Here’s Marco Rubio, a fresh-faced kid who’s seen as trying to jump into the race and relatively inexperienced. He’s got more of a baby face than Ted Cruz, who's got a ‘quiet assassin’ type [of face].”

Truth be told, political experts say, most voters do not study the details of policy positions of presidential candidates, and debates tend to veer off into sound bites and demeanor. So gut instincts end up playing a big part in how the public feels about a candidate.

“People look for clues and cues to tell them whether a candidate is dependable, experienced, whether they could understand you,” Jillson said. “Middle-aged and older voters tend to equate a few wrinkles with experience. They look to see who has the seasoning to be president. Some people think Rubio is not seasoned enough, too young” to handle someone like Vladimir Putin.

Mindful of the criticism, Rubio has made a point of portraying his age, 44, as an advantage over his much older rivals in both parties.

The Florida lawmaker has presented himself as a president who would bring the country into the future, and bring fresh ideas and new ways of doing things to the Oval Office and to Beltway politics.

In what many have seen as a swipe at both former Florida Gov. Jeb Bush, one of his GOP rivals and former mentor, and former Secretary of State Hillary Clinton, running on the Democratic side, Rubio has condemned presidential candidates he characterizes as throwbacks to yesterday’s way of doing things, and as mere continuations of political legacies.

On the Democratic side, Clinton is 68, Sen. Bernie Sanders of Vermont is 74, and former Maryland Gov. Martin O’Malley is 53. In the GOP field, Donald Trump is 69, Ben Carson is 64, Bush is 62, former Hewlett-Packard CEO Carly Fiorina is 61, Sen. Rand Paul of Kentucky is 53, New Jersey Gov. Chris Christie is 53, Ohio Gov. John Kasich is 63, Huckabee is 60, and former Sen. Rick Santorum of Pennsylvania is 57.

Vander Plaats said while Rubio’s boyish looks may be working against him, he also has been hurt by his former highly visible role in the so-called “Gang of Eight” – a bipartisan group of senators who drafted and pushed a comprehensive immigration reform bill in 2013 that called for tighter enforcement but also for allowing perhaps millions of undocumented immigrants to legalize their status.

Many conservatives grew angry with Rubio over his support for legalizing undocumented immigrants, denouncing it as support for amnesty.

Rubio argued back then that it was not amnesty, which essentially is a pardon, but in fact was an earned legal status that required an extensive set of criteria – such as paying fines, not having a police record, paying taxes, and a wait of more than 10 years before immigrants could apply to become naturalized citizens.

Vander Plaats said he spoke with Rubio at the time, and advised him to “break from the Gang of Eight.”

“You look like you’re following them,” he recalled telling Rubio in referring to Democrats. “People were upset about the amnesty part. Bad company corrupts good character. He could have been conservative by saying ‘We need to have border security, we need a 21st century process so that we know who’s in the country.’”

Vander Plaats said he is not altogether against finding a way to bring certain undocumented people out of the shadows.

“For those who are here illegally, I think we can do it,” he said, but added that Rubio should have focused on addressing security and enforcement weaknesses in the immigration system before pushing for a path to legal status.

“When he was doing that, the immigration [issue] was different than it is today,” Vander Plaats said.

That is what Rubio has been saying when GOP presidential rivals and critics raise his Gang of Eight involvement.

In GOP debates and media interviews, Rubio says the debate about immigration, and the priorities regarding the issue, have changed since 2013.

No longer is it just about dealing with the 11-12 million people here illegally, he says, it is a national security issue, given how Islamic State militants are trying to exploit visa laws to enter the United States.