As Trump soars in polls, Marco Rubio, one-time leader of GOP field, slides further

Republican presidential hopeful and U.S. Senator Marco Rubio (R-FL) answers questions from the audience at the Council on Foreign Relations on May 13, 2015 in New York City. (Photo by Andrew Burton/Getty Images)

Republican presidential hopeful and U.S. Senator Marco Rubio (R-FL) answers questions from the audience at the Council on Foreign Relations on May 13, 2015 in New York City. (Photo by Andrew Burton/Getty Images)  (2015 Getty Images)

It was only a few weeks ago that he seemed invincible.

Sen. Marco Rubio launched a presidential campaign with a speech that still is viewed as the most eloquent and moving of all those delivered so far by the  nearly two dozen candidates who have declared their run.

The telegenic, smart, polished junior Republican from South Florida ranked high in the polls of likely GOP voters. He led the GOP, as a matter of fact, with 14 percent in June polls.

But in July, he started sliding, showing the most significant drop of any GOP candidate. This weekend, an NBC/Wall Street Journal poll of GOP likely voters showed Rubio barely making it among the top 10, with only 5 percent choosing him.

Real estate mogul Donald Trump trounced all others, with nearly 20 percent. Wisconsin Gov. Scott Walker came in second with 15 percent, and former Florida Gov. Jeb Bush was third with 14 percent.

Rubio came in eighth, behind Texas Sen. Ted Cruz, former Arkansas Gov. Mike Huckabee and Sen. Rand Paul of Kentucky.

Many experts say that Rubio, who constantly made headlines with his thoughts – whether through email blasts or Sunday morning news shows – on everything from immigration to foreign policy, has fallen to the margins of public attention because of Trump.

“First and foremost, what’s happened to him and a lot of these candidates is Donald Trump,” said Gregory Valliere, chief political strategist at the non-partisan Potomac Research Group, in an interview with Fox News Latino. “Trump has made it really difficult for a lot of Republican candidates to gain traction. Marco Rubio hasn’t really done anything that makes him stand out from the pack. He hasn’t said anything outrageous.”

Rubio also has maintain a relatively low profile as far as responding to Trump, as other candidates – such as former Texas Gov. Rick Perry and South Carolina Sen. Lindsey Graham, who have assailed the tycoon – have done.

“He hasn’t taken him on like Perry or Walker or even Cruz, who shamelessly sucked up to Donald Trump,” Valliere said. “Marco Rubio wants to talk about his candidacy and his proposals.”

GOP strategist Liz Mair told Business Insider that Rubio’s pet issue – foreign policy, on which he is a hawk and often blasts the Obama administration as being too weak – has often taken a back seat in the news recently to other topics such as gay marriage, police-involved shootings and the Affordable Care Act.

“For Rubio, it’s better for people to focus on foreign policy,” Mair said.

The Aug. 6 debate can be a game-changer, at least until the debate that follows it.

While many experts expect that many people will tune in to watch Trump, and how he plays off the other candidates, it will be a chance for contenders like Rubio – who remains largely enigmatic to many GOP voters nationwide – to resonate with the audience and regain ranking.

In his favor is a consistently solid favorability rate, meaning many Republicans find him likable. Rubio has a favorability rating of 49 percent among Republicans and Republican-leaning independents; his negative ratings among them is 15 percent.

The remainder largely said they don’t know enough about him, giving him room to appeal to more people.

Jeb Bush has a favorable rate of 51 percent, with 30 percent saying they dislike him.

Trump, on the other hand, has a 49 percent unfavorability rate, and 42 percent favorability.

His supporters are rather ironclad, though, experts say.

“Among the party base, he’s a force to be reckoned with,” said Lee Miringoff, director of the Marist Institute for Public Opinion in New York, according to McClatchy news service.

But the good news for candidates not flying high at the moment is that it is still very early in the election cycle.

The nominee for each party will not be chosen for almost another year, they say.

And anything can happen in a year in politics.

“The Iran nuclear deal is being discussed now. Rubio will play a pretty commanding role on that one,” said Valliere. “He also has raised a decent amount of money. It’s too early to dismiss him. I think he’ll have staying power.”

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Elizabeth Llorente is Senior Reporter for FoxNews.com, and can be reached at Elizabeth.Llorente@Foxnews.com. Follow her on Twitter @Liz_Llorente.