5 challenges Trump faces in winning the Republican presidential nomination

After a series of controversial statements, underwhelmingdebate performancesand a poorly reviewed Saturday Night Live appearance to boot, DonaldTrump still remains at the top of the polls, confoundingthe political punditswho long ago predicted his campaign would crash and burn.

TheIowa caucuses, the first time Republicans begin to selecta candidate, aren't until February 2016, meaning Trump and his policies and statements still have time toundergofar more scrutiny. But those aren't the only challenges Trump could face in winningthe Republican presidential nomination.

1. No GOP establishment endorsements so far

The political history of presidential nomination races inboth the Republicanand Democrat partiesteaches that the eventual candidateis one who has some, if not the majority of,backing from the so-calledpartyestablishment.

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Former president Ronald Reagan, for example,had virtually no support when he made his unsuccessful run against Gerald Ford in 1976, but had managed to get somebackingfour years later when he won. Indeed,the last Republican to carry the party's nomination with little supportfrom the establishment was Arizona senatorBarry Goldwater in 1964, whowent on to a crushing defeatat the hands of Democratic nominee and incumbent presidentLyndonJohnson.

As forTrump, according to the statisticalanalyticswebsiteFiveThirtyEight.com, he hasno endorsements from any Republican governor, senator or representative in the House.

Whileendorsements may not be adirect indicator of a candidate'schances of winning, they doserveas an indirectsignalof the potential for candidates toconsolidatesupport through donorconnections and party officials in various states, says GeoffreySkelley, a political analyst at theUniversity of Virginia Center for Politics.

"It's tough to see him winning without any support within the party,"Skelley said. "I don't think you'd ever see a governor or senator endorsing Trump.Maybe one or two members of the House might."

"Basically, you can't take the endorsements in and of themselves as a sign that someone's going to win. But you can look at it as a signal of general support within the party and all that comes with that."

2. Party establishment could try to burn hiscandidacy

If Republican Partyelders become concerned Trump has a real shot at winning, and that his nomination could cost the party the White House, they could launch a blistering advertising and ground campaign against him.

Or in what mightbe a much more constructive approach they could direct all their resourcesto one of the other candidates,saysJulianZelizer, professor of history and public affairs at Princeton University.

"Nowthey'resplittingtheirmoneyand time. But if they really focus in on one person, the establishmentstill has a lot of powers.You can read aboutendorsementseveryday, youcanseemoney really move in thedirectionof one candidate,and that often will create a dynamic that couldshrink Trump's [lead]."

Nate Silver, founder of FiveThirtyEight, pointed out back in August that the Republican Party's delegate selection rules work in favour of establishment-backed candidates and against candidates like Trump.Delegates in some states are not formally pledged to the candidate who won that state, he noted, while about seven per cent of delegates to the Republican National Conventionare party leaders.

"Much of the party's influence consists of what you might call 'soft power,'the ability to influence outcomes by persuasion rather than coercion," Silver wrote."But the party also has some 'hard power': It literally makes the rules."

3. High poll numbersmay nottranslate into votes

Trump may be leading in the polls, but it remains to be seen whetherthose supporters, some of whom have only just become politically engaged because of their attraction to the straight-shooting real-estate mogul, will show up at the primaries or caucuses to vote.

His support, according to polls,is quite broad, cuttingacross a large swath of the party but comingfrom more moderate Republicans, Skelley says.

"Who is more likely to show up in an actual Republican primary or caucus someone who is a diehard committedconservativeor very conservative Republican,or someonewho is moremoderate or has only become engaged because of Trump?" Skelley said.

"And you wonder if Trump'spoll numbers slip abit, does that end up sort of turning offthe peoplewho were supporting him who aren't typically that engaged intheprocess? Will they show up and vote?That's an open question."

4. Trump'suntested ground game and endurance

At acertainpoint, the Republican nomination racewillturninto a contest of organization and not simply public relations, Zelizer says.That means Trump will need an extensive get-out-the vote ground game in all of the states to remain competitive.

"Aswe get into Iowa and we get into New Hampshire and all the other primaries, you're goingto need a very formidable grassroots organization to get him through those contests," Zelizer said. "He's untested at this point.He's hired some serious people in Iowa, but Idon't know.Can he sustainthat throughthemany primaries that are required to win?"

As well, Zelizer questioned whetherTrump has theendurance to slog through the primaries and "deal with the day to day and very brutal process of campaigning in all these states."

"In the next stage, it gets much harder than giving some speeches and going on national television," Zelizer said.

5.Ben Carson's support would move to Trump? Not necessarily

According to some polls, neurosurgeonBen Carson, another so-called anti-establishment candidate,is about even withTrump at around 25 per cent support in the Republican race.It's possible Trump could pick up much of Carson'ssupport if Carson were to drop out, meaning Trumpcould still be a force to reckon with even if the establishment coalescedbehind another candidate.

But Carson'ssupporters tend to be more conservative, andcome more from theevangelical base,than Trump's. It's not at all a given that those who support Carson, who first came on the national scene because of his opposition to Obamacare, would slide over to Trump, who has in the past praised the idea of national health care and tax increases.

"It's not clear [Carson's support] hasthe same kind of anti-establishment fervour," Zelizer said.