COLUMBIA, S.C. — Throughout Donald Trump’s improbable rise to becoming the frontrunner for the Republican presidential nomination, he has faced doubters.

Political observers and pundits questioned his chances. Media outlets dismissed him as an entertainer. There was speculation about whether Trump, a real estate mogul and reality television star, actually expected to get this far. Even some of his competitors refused to consider his campaign something real and to be taken seriously.

But according to top aides, Trump always believed he had the potential to reach the White House and has wanted to win the race. As evidence of Trump’s confidence and commitment, one source close to the campaign told Yahoo News that at the beginning of the year, Trump signed some members of his political staff to yearlong contracts that extended through Election Day.

And now, after his decisive win in South Carolina, one of the architects of Trump’s Palmetto State victory says the brash New Yorker is positioned to “handily” win the Republican nomination.

Ed McMullen, one of Trump’s campaign co-chairs in South Carolina, told Yahoo News he believes the primary has now become a “single-man race,” with Trump too far beyond his top rivals — Sen. Marco Rubio, R-Fla., and Sen. Ted Cruz, R-Texas — for them to catch up.

Trump didn’t just win the primary in South Carolina on Saturday. He crushed the competition, coming in over 10 points ahead of second place and carrying 44 of the state’s 46 counties.

“It’s now starting to coalesce. Republicans are starting to realize from every demographic group that this guy is Reagan-esque. He’s a serious leader and he is something different that this country needs now,” McMullen said. “It is a single-man race. They have to beat him because he has got a message that people are starting to join together for and support. And I think he will win handily as we go forward.”

It wasn’t supposed to be this way. Former Florida Gov. Jeb Bush was the choice of many in the Republican leadership and the donor class. Bush, whose father and brother both served as president, dismissed Trump as “an entertainer.” In the early days of the campaign, Bush’s massive fundraising haul was supposed to scare away potential rivals. Instead, as Trump relished his victory on Saturday, Bush announced the suspension of his campaign. The Bush family dynasty fell.

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Trump, with his socially liberal streak, political inexperience and divisive stance on immigration, is essentially a nightmare for the old Republican guard. With Bush’s departure, there are only two candidates left who are seen as palatable for the GOP’s establishment: Ohio Gov. John Kasich and Rubio. Yet Kasich is essentially running on fumes after landing in single digits in two of the first three voting states. And Rubio is battling with Cruz — who, like him, has roots in the party’s conservative tea party wing but, unlike him, has run as a hard-right religious conservative in 2016 — to emerge as Trump’s sole rival.

With this landscape, McMullen sees an easy Trump victory. He predicted that Trump will pick up support due to the fact he’s already out in front while Cruz and Rubio duke it out. McMullen also suggested Trump’s and Cruz’s performances so far show the majority of Republicans don’t want a more establishment leader.

“Look, anyone who looks at elections knows it’s all about math, right? When you have seven or eight people in a race, you have a ceiling. People drop out, your ceiling goes up,” McMullen said. “He’ll continue to show growth. What we do know is at least two-thirds of Republican primary voters are tired of the nonsense, tired of the status quo, and they want serious reform. Those two-thirds are not going to find reform or change in Marco Rubio.”

So where does this leave the Republican Party’s old guard? McMullen argued that the GOP establishment is on its last legs.

“I mean, I am the establishment. … What is the establishment now? I think the establishment is tired of Washington as usual and the money changers … the political hacks, and the consultants, and the people making money off of the party are the ones trying to keep their last vestiges of strength and power,” McMullen said. “The fact of the matter is Mr. Trump has slapped them in the face and told them it’s time to change. And they are struggling and scampering in every little hole to figure out how to keep their control and their money flow.”