By Noah Barkin

BERLIN (Reuters) - Britain's Nigel Farage will be at the Republican convention in Cleveland next week to dole out lessons from his Brexit campaign. So will Geert Wilders, leader of the Dutch Freedom Party, who sees Donald Trump as an ally in his crusade to prevent an "Islamization" of the West.

But some mainstream European conservatives who have traveled to previous Republican gatherings admit in private to being horrified at the prospect of a Trump presidency and are boycotting this year's spectacle, which runs from July 18-21.

Many of the center-right politicians making the trip from Europe admit to secretly rooting for Trump's Democratic rival Hillary Clinton. They are going not to support Trump, they say, but to witness "the Donald" in the flesh and experience what is shaping up to be one of the rowdiest, most unpredictable Republican conventions in decades.

"We will see whether Trump can reinvent himself at the convention," Guenter Krings, a senior lawmaker from German Chancellor Angela Merkel's conservatives who will make the trip with just one other Bundestag colleague, told Reuters. "One can only hope so, but I have my doubts."

Politicians from around the world are invited every four years to attend the Republican and Democratic conventions as part of a formal visitors program that includes briefings with campaign managers, pollsters and foreign policy experts. They also spend time in the main convention hall.

This year, nearly 120 politicians from center-right parties outside the United States are expected, roughly half of them from Europe.

Trump's candidacy has presented European parties with traditional links to U.S. Republicans with a dilemma. Many view his positions -- on issues like immigration, security, trade and the divisive Brexit question -- as closer to those of upstart far-right parties whose rise they are struggling to contain.

EPP BOYCOTT

Four years ago, when Mitt Romney was the Republican candidate, the center-right European People's Party (EPP), the largest grouping in the European Parliament, sent a delegation of eight members to Tampa.

This year there will be no EPP delegation because no-one from the group expressed an interest in attending.

Despite the wariness, there is a reluctance to criticize the famously thin-skinned Trump too openly, in part out of fear he could win the election in November and punish his foreign critics, as he threatened to do when David Cameron, the recently departed British prime minister, slammed Trump's proposal for a temporary U.S. ban on Muslims as "divisive, stupid and wrong".

While the conservative mainstream is leery, Europe's array of nationalist, anti-immigration parties seem fascinated by Trump, even if some are uneasy about embracing the New York real estate businessman, who has vowed to "make America great again".

A spokesman for Britain's UKIP party in Brussels confirmed that Farage would attend to "give them some lessons" on Brexit. And Wilders told Reuters that he would be in Cleveland and wanted to meet Trump there.

"I hope Donald Trump becomes America's next president," Wilders said. "Such a victory would be good for Europe. America and Europe are threatened by the same danger of cultural relativism and Islam."

Austria's far-right Freedom Party, France's National Front and the Alternative for Germany (AfD) are not planning to send anyone to Cleveland, the parties said.

"We are neutral," Freedom Party leader Heinz-Christian Strache told Reuters when asked if he supported Trump. "He defends American interests. He is saying some things that need to be said."Also absent will be Italy's Northern League, whose leader Matteo Salvini describes himself as an enthusiastic Trump fan but learned earlier this year that the admiration was not necessarily mutual.

Salvini traveled to Pennsylvania in April to attend a Trump rally. He was photographed shaking hands with the Republican candidate and his party said Trump had endorsed Salvini to become Italian prime minister.

But a month later, Trump denied in an interview ever having met Salvini and said he had no interest in doing so.

SHOCK AND HORROR

Eirik Moen of the International Democrat Union, which coordinates trips to the convention by center-right politicians across the world, said that numbers could be dented this year because countries like Britain, Spain and Australia are in the midst of political transitions, making it difficult for some to travel.

Still, eight British Conservatives who sit in the European Parliament are expected to be in Cleveland.

On the continent, France's main center-right party, les Republicains, is not sending a delegation. And the contingent from Merkel's Christian Democrats (CDU) is unusually small.

In Germany, political debate is conducted in a sober, reserved manner and personal attacks are rare, even during election campaigns. Hence the mix of shock and horror at the tone of the U.S. race.

Trump has dismissed Merkel as "insane" for letting hundreds of thousands of refugees into the country. And his idea to build a wall along the Mexican border elicits shudders in Berlin, a city that was divided by its own wall for nearly three decades and will celebrate the 27th anniversary of its fall a day after U.S. voters cast their ballots on Nov. 8.

"Many of Donald Trump's positions -- for example his stance toward Muslims, toward immigration from Mexico and gun ownership -- are simply not compatible with the positions of the conservatives and other parties in Germany," said Juergen Hardt, a CDU lawmaker who also acts as the government's coordinator for transatlantic relations.

Hardt said he is planning to go on vacation with his family next week and won't be able to attend the convention.

(Additional reporting by Alastair Macdonald, Crispian Balmer, Kirsti Knolle, Ingrid Melander, Julien Toyer, Anthony Deutsch and Robert-Jan Bartunek; Editing by Sonya Hepinstall)