(Corrects length of Simpson service, spelling of Scott Reed)
By Andy Sullivan
BOISE, Idaho, May 18 (Reuters) - On a recent Saturday, foot soldiers in the Republican Party's civil war fanned out across a neighborhood of winding streets and manicured lawns to beat back the Tea Party.
They did not meet much resistance. Over the course of two hours, dozens of residents said they planned to vote in the May 20 primary election for Idaho Representative Mike Simpson, a veteran lawmaker with a reputation for pragmatism. None said they would back his Tea Party challenger Bryan Smith.
As he heads for a likely victory on Tuesday, Simpson has plenty of help. The U.S. Chamber of Commerce, former Republican presidential candidate Mitt Romney and the National Rifle Association have joined with groups representing hospitals, dentists and home sellers to try to return him to Congress for a ninth term.
The push is part of a wider drive by so-called establishment Republicans to back mainstream candidates and prevent Tea Party figures like Smith from winning primaries ahead of the Nov. 4 midterm elections.
They fear a repeat of some key races in previous congressional elections when Tea Party insurgents alienated independents and lost to Democrats. Party elders still smart at the loss of a U.S. Senate contest in Missouri in 2012 which was blamed on comments about "legitimate rape" by Republican candidate Todd Akin.
While the Tea Party movement has helped Republicans win significant spending cuts and thwart some of President Barack Obama's agenda, it also forced a messy shutdown of the U.S. government in 2013 that was unpopular with voters.
In Idaho, organizations that have spent more than $2 million to back veteran Republican Simpson look likely to get a return on their investment, thanks to a deluge of TV advertising and months of door-knocking by workers like Stacey Barrack.
"We talk about his conservative record and everything he's done in office, and it's worked," said Barrack, 28. She entered details of each doorstep encounter into her smart phone.
Working closely together, the pro-Simpson groups have created a shadow campaign of sorts that mirrors, and outspends, the functions of the candidate's own campaign, sharing data from door-knocking efforts and telephone surveys to determine what messages work best and coordinating their ad buys to ensure they are not duplicating their efforts.
"Everybody does their own product, but we try to make sure everything marries up," said Scott Reed of the U.S. Chamber, which was the first group to run ads for Simpson and has spent the most money in the race.
One of its TV ads features an endorsement for Simpson from Romney, still a pillar of the Republican establishment despite his 2012 loss to Obama.
Barrack and the other door knockers have been hired by Defending Main Street, a group funded in part by the Laborers Union and the Operating Engineers Union, which unlike most other labor unions give money to both political parties.
As a senior member of the House Appropriations Committee, Simpson is the kind of influential, practical-minded Republican who professional associations are keen to cultivate. Lobbyists say his deep knowledge of a wide range of issues, from nuclear energy to forestry, makes him a valuable ally.
The NRA has mailed out flyers for him and the National Association of Realtors has paid for online video ads. The American Hospital Association and the American Dental Association have paid for polls, mail and TV and radio spots.
NORTH CAROLINA, KENTUCKY
Elsewhere, the Republican establishment is also gunning for the Tea Party as it seeks to present electable candidates who can win control of the whole of Congress and make Obama's life difficult for the rest of his term. Republicans need only a gain of six seats to take the Senate back from the Democrats.
The U.S. Chamber has emerged as one of the biggest spenders so far this year, pumping $12.3 million into the campaigns of 20 business-friendly candidates from Alaska to Florida.
The Chamber and Defending Main Street helped Representative David Joyce fend off a Tea Party-backed challenger in Ohio this month, and also picked up a win in North Carolina, where it worked with the NRA to help state assembly speaker Thom Tillis win a primary battle for a Senate seat.
In Kentucky, Senate Republican Leader Mitch McConnell is holding a wide lead over Tea Party challenger Matt Bevin, thanks in part to advertising, polling and other campaign efforts by the NRA, the Chamber and a handful of other business groups.
The Tea Party's popularity has ebbed in recent years. A Gallup poll released this month found that 41 percent of Republicans considered themselves Tea Party supporters, down from 61 percent in November 2010.
Still, the Tea Party showed it is still a force in the congressional elections when Ben Sasse, a candidate associated with the movement, won a Nebraska Senate seat primary.
And no matter the outcome of this year's primaries, the movement has pushed the Republican Party as a whole farther to the right.
The fiscally conservative Club for Growth advocacy group which often backs Tea Party hopefuls, sees less of a need this election cycle to spend money to get its low-tax message out.
"Now we're talking about how much less we're going to spend as opposed to how much more we're going to spend," said spokesman Barney Keller. "That's a sea change from where we were."
Since 1998, Simpson has forged a reputation as a jovial dealmaker who is willing to work with Democrats in order to look after the interests of a sprawling district that encompasses jagged mountain ranges, Mormon farm country, and the microbreweries and bicycle shops of Boise. A close ally of House Speaker John Boehner, Simpson regularly votes for budget deals that are spurned by Tea Party-backed lawmakers.
Simpson's opponent Smith, an Idaho Falls lawyer, has also received outside backing from small-government groups like Freedomworks and the Club for Growth, which in previous cycles helped elect Tea Party stars like Texas Senator Ted Cruz.
Smith criticizes Simpson's support for compromises like the 2008 Wall Street bailout and the 2013 bipartisan budget deal.
"If you're like Congressman Simpson and you vote for a $700 billion bailout, you can naturally expect to get the support of constituents who are the beneficiaries of that," Smith said.
Simpson is likely to win the primary on Tuesday, according to independent forecasters, and is strongly favored to go on to defeat his Democratic opponent in November.
Groups that back Simpson and other pragmatic Republicans in the primary are careful to portray them as reliable conservatives rather than moderates.
As she makes her rounds in the streets of Boise, Barrack distributes flyers that emphasize Simpson's A+ rating with the NRA, his 100-percent rating from National Right to Life, an anti-abortion group, and the more than 40 times he has voted to repeal Obamacare.
Simpson said national business groups were supporting him because their members in the state give him high marks.
"The NRA has members all over Idaho. If those members don't support me, the NRA doesn't support me. If business in Idaho doesn't support me, the U.S. Chamber doesn't support me," he said in an interview. (Editing by Alistair Bell and Ross Colvin)