U.S. Republican presidential candidate Donald Trump speaks at a campaign event in Buffalo, New York, U.S., April 18, 2016. REUTERS/Carlo Allegri

By James Oliphant

WASHINGTON (Reuters) - He’s a successful businessman making his first foray into politics. He wants to secure the southern border with Mexico and dump global trade deals. And like Donald Trump, he wants to be the Republican establishment's worst nightmare.

He’s Paul Nehlen, who has emerged as a Republican rival to Paul Ryan, the speaker of the U.S. House of Representatives, in his home district in Wisconsin. Ryan, the most powerful elected Republican in the country, is the epitome of that establishment.

While many Republican operatives fear Trump’s tumultuous presidential bid will hurt House and Senate races this year, a handful of candidates like Nehlen are embracing Trump’s message, although not always the man himself.

A loss by Ryan or another high-profile incumbent would send shockwaves through the party and could prompt fellow establishment Republicans to rethink positions on advocacy of free trade and support for immigration reform.

In New Hampshire, where Trump won the nominating contest, or primary, by 20 points, Jim Rubens, a former Republican state senator who has endorsed the billionaire businessman, is attempting to unseat incumbent Senator Kelly Ayotte.

In Arizona, another state Trump won, state senator Kelli Ward is challenging veteran Senator John McCain by talking up securing the border and courting voters at Trump rallies, although she has not endorsed the presidential candidate.

The dynamic of outsiders challenging establishment candidates is one that has been taking place within the Republican Party for several election cycles, but in Trump, the insurgency has a new, more popular face.

Beyond the races in Wisconsin, Arizona and New Hampshire, it has played out in states like Alabama, where Jonathan McConnell greeted voters outside a Trump rally in Huntsville in a bid to unseat veteran Senator Richard Shelby, and North Carolina, where Rep. Renee Elmers, facing a challenge to her seat, said she had voted for Trump in that state’s presidential primary.

But, unlike the anti-establishment wave that swept Tea Party-aligned lawmakers into Congress in 2010, these candidates do not appear to represent a new anti-Washington groundswell, suggesting there are limits to Trump’s brand of politics, which at times runs deep against Republican orthodoxy on issues such as trade and taxes.

“I been surprised that there haven’t been any Senate or House incumbents that have lost primaries as a result of a groundswell of support for Donald Trump or Ted Cruz,” said Nathan Gonzales, an independent analyst on congressional races in Washington.

Shelby, for example, soundly defeated challenger McConnell in Alabama's Republican primary race in March.

“Other candidates could try to replicate his message, but no one can replicate Donald Trump,” Gonzales said.

Still, many analysts and political operatives believe Republicans unhappy with Trump as the nominee won't vote in the general election, potentially causing Republicans to lose their congressional seats to Democratic contenders.

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It would be easy, to dismiss Nehlen’s challenge to Ryan, who is popular among Republicans inside and outside of Wisconsin and has a large campaign warchest. A poll last month by Marquette University Law School showed him with more than an 80 percent approval rating among Wisconsin Republicans.

But less than two years ago, Eric Cantor, then the No. 2 Republican in the House of Representatives, was also considered a rising Republican star. He suffered a shock defeat in the 2014 congressional elections in Virginia to an obscure conservative college professor, David Brat. Cantor, too, had looked unbeatable.

“That race is what reinforced in my mind this is possible,” Nehlen said in an interview.

Trump has frequently criticized Ryan for the $1.1 billion budget deal struck with Democratic President Barack Obama last December. And Ryan’s support of trade deals such as the Trans-Pacific Partnership and comprehensive immigration reform is squarely at odds with Trump’s positions.

Nehlen said those two issues motivated him to mount his challenge to Ryan, although he stopped short of saying he embraced Trump's candidacy.

Rubens, who is challenging Ayotte in New Hampshire, isn’t as restrained. ”I admire his independence,” Rubens told Reuters.

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