Donald Trump is not your typical Republican, but the party of the US centre-right is due to throw its support behind the unlikely politician for the highest US elected office at its convention in Cleveland, Ohio, next week.

Trump entered a crowded field of Republican presidential candidates last summer as the clear underdog against known names like Florida Governor Jeb Bush and up-and-coming first-term senators, Marco Rubio and Ted Cruz.

Trump was seen as something of a joke for late-night comics, but US media were drawn to his brash statements in interviews and off-the-cuff social media comments. Trump sought to appeal to working-class white voters frustrated with the Republican establishment and quickly built a loyal following that appeared at large rallies where he vowed to "Make America Great Again."

Trump has continued to double down on that message since securing the party's nomination, declaring this week, "The disconnect in America is deep. There are two Americas; the ruling class and the groups it favours and then everyone else."

Wins in early primary states in February and March gave Trump key momentum and his surprised challengers were unable to regroup as they split the vote of more traditional Republican supporters and left the race one by one.

On Monday in Cleveland, Trump's ascendancy will be nearly complete with just one more jewel to be won - the White House.

Thousands of Republican party loyalists and officials will arrive in Cleveland, among an estimated 50,000 visitors, including 15,000 members of the media.

The convention centres around a vote by the party's 2,472 delegates to officially pick its presidential nominee. The winner needs 1,237 delegates and Trump cleared that hurdle in May.

The four-day event will culminate in Trump's acceptance of the nomination in a televised speech on Thursday evening. The rest of the convention will feature approval of the party's positions and speeches by party loyalists and other candidates, including Trump's vice presidential pick.

A partial list of speakers released Thursday show Trump's family, including his four adult children and wife Melania, will play a prominent role. Other speakers include former Trump rival Senator Ted Cruz and the Republican leaders of Congress, but fewer lawmakers than usual appear on the list.

Not everyone is thrilled about Trump however, with some within the party vowing never to support the candidate whom they fear will damage the prospects of other Republican candidates and whose views do not reflect party orthodoxy.

A small group of delegates is pushing to change party rules ahead of the convention to make it easier for delegates to vote against Trump. They hope to pass a measure that would allow delegates to vote their conscience rather than binding them to the voting results in their states.

Republican leaders are opposed to such a move, but the anti-Trump forces hope to push the issue to the convention floor or cause other disruptions to show dissatisfaction with Trump.

"We are saddled with a presidential candidate who holds none of the qualities of our party's greatest leaders. He lacks common decency, respect of the Constitution, and the temperament of someone fit to be Commander-in-Chief," one group calling itself Free the Delegates writes on its website.

Though it will be difficult for those anti-Trump forces to prevail, they have some high-profile company. Former president George W Bush and George HW Bush along with previous nominees John McCain and Mitt Romney will skip the convention. Romney has been especially vocal against Trump.

The show of disunity is seen as an opportunity by Democrats and their presumptive nominee Hillary Clinton.

"The more Trump is Trump, the more Republicans and conservative commentators criticize and condemn his candidacy," the Clinton campaign said in an email Wednesday. "With the GOP convention less than a week away, Republicans are still not lining up behind Trump."

Several top corporate sponsors, such as Coca Cola, have also shied away from supporting the convention this year amid concerns about being tied to Trump's controversial statements and anti-trade stance.

Trump meanwhile is focused on choosing his vice presidential running-mate and told the Wall Street Journal he has narrowed the list to Indiana Governor Mike Pence, New Jersey Governor Chris Christie and former speaker of the House Newt Gingrich, who was a key nemesis of Bill Clinton during the 1990s.

Trump has said he wants a vice presidential pick experienced in lawmaking to make up for his own lack of experience, but is also weighing other concerns including how their personalities mesh with his own brash persona.

Trump hopes to capitalize on Clinton's recent woes over her use of a private email server as secretary of state and has also tried to tie her to unpopular corporate interests on Wall Street.

An average of national opinion polls show Clinton with a lead over Trump, but he has recently narrowed the gap in a number of key states. The race is too close to call in Florida, Ohio and Pennsylvania, all considered crucial to winning the presidency, according to a Quinnipiac University poll released Wednesday.

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