By Alistair Bell

KINGSTON, New Hampshire, Aug 7 (Reuters) - As President Barack Obama considers sidestepping Congress to loosen U.S. immigration policy, a Reuters/Ipsos poll shows Americans are deeply worried that illegal immigration is threatening the nation's culture and economy.

Seventy percent of Americans - including 86 percent of Republicans - believe undocumented immigrants threaten traditional U.S. beliefs and customs, according to the poll.

The findings suggest immigration could join Obamacare - the healthcare insurance overhaul - and the economy as hot button issues that encourage more Republicans to vote in November's congressional election.

With Congress failing to agree on broad immigration reforms, Obama could act alone in the next few weeks to give work permits to up to 5 million undocumented immigrants and delay some deportations, according to media reports.

Hispanic and liberal voters would welcome that, but the online survey suggests much of the rest of the nation may not.

Despite arguments from the White House and groups like the U.S. Chamber of Commerce that legal immigration benefits business, 63 percent of people in the online survey also said immigrants place a burden on the economy.

While the economy and Obamacare remain the key concerns of voters, immigration has become more of an issue in recent months because of intense media coverage of a surge of illegal migrants, including tens of thousands of children, flooding into the United States from Central America.

Even 2,000 miles (3,200 km) from the U.S.-Mexico border, the immigration debate is catching fire in New Hampshire. Republican Senate candidate Scott Brown last week launched a TV ad attacking Democratic Senator Jeanne Shaheen for "pro-amnesty policies" on immigration which he says she shares with Obama.

While Brown is trailing the Democrat in polls by around 10 points and still needs to win his own primary, it was the first ad by a major Senate candidate to focus on immigration and the crisis of children on the border.

New Hampshire does not have a large Hispanic immigrant population, but conservatives' concern about the burden on local services from Somali, Sudanese and Bhutanese refugees has simmered for several years.

When Brown talked to voters at a Veterans of Foreign Wars hall in the town of Kingston last week, most audience questions were about immigration.

One woman asked how authorities could ensure any child migrants from Central America sent to New Hampshire were free from disease, although it was not clear at the time if any had actually been resettled in the state.

TOUGH POLL NUMBERS

In Denver, Reuters/Ipsos poll respondent Tom Vanderbur, 72, criticized Congress for going into summer recess after failing to pass border security measures but said he was not convinced that Obama should act unilaterally on immigration.

"I don't think that he has a right to just make those kinds of decisions on his own," said retiree Vanderbur, a registered Democrat in a state where another U.S. Senate seat is being hotly contested.

Vanderbur was among the 45 percent of people in the poll, carried out between July 15-22, who said the number of immigrants legally allowed to enter the country should be reduced.

Only 17 percent thought more legal immigrants should be allowed to come to the United States. Thirty-eight percent said the number should stay the same.

"If Obama starts using executive orders to grant citizenship or to stop deportations I think he gives Republicans a big opening," said Jennifer Duffy, of the Cook Political Report analyst group. "It'll be about the issue at hand, immigration, but it also feeds into this Republican narrative of overreach, of sort of abusing his power."

Ahead of the elections, Republicans have accused Obama of creating a "legacy of lawlessness" by threatening to bypass a deadlocked Congress, and last month House Republicans voted to file a lawsuit against him on charges that he overstepped his authority in carrying out key parts of Obamacare.

RISKS OF GOING SOLO

Obama acting alone on immigration could make Republicans more likely to go out and vote in November, said Ipsos pollster Chris Jackson.

"It'll certainly contribute to energizing Republicans," Jackson said. Independent voters are also wary of new waves of immigrants but they tend not to turn out heavily at midterm elections.

Beyond New Hampshire, immigration has come up as a contentious issue in the Colorado and Arkansas Senate races and the Arizona governor's election.

A separate Reuters/Ipsos online poll shows that voters see immigration as the third most important problem facing the nation. Respondents in a Gallup poll in July cited immigration as the No.1 problem, ahead of the economy.

A Republican candidate for New Hampshire state senator, Eddie Edwards, said seven out of 10 voters voice worries about immigration and refugees when he canvasses door to door.

"If President Obama issues a jarring set of executive actions on legalization he could be handing the Senate to the GOP, including putting New Hampshire truly in play," said Republican strategist Ford O'Connell.

White House spokesman Josh Earnest said this week he would not discuss details of how Obama might use his executive authority on immigration, but any measures Obama takes "may be in a position to mitigate some of the problems of our broken immigration system."

Nationally, immigration has long been seen as a vote winner for Democrats and Obama, who won more than 70 percent of the Latino vote in his 2012 re-election. Republicans' refusal since then to approve immigration reform has further alienated Hispanics, but Latino voters do not traditionally vote in large numbers in congressional elections, blunting the Democrats' advantage with that group in November.

Opposition to illegal immigration is higher in New England than in much of the rest of the country, according to the Reuters/Ipsos poll of mid-July. Seventy-six percent of people in the region said undocumented immigrants threaten American beliefs and customs, compared to 70 percent nationally.

Bill Roy, a retired postal work from Manchester, New Hampshire, has voted for Democrats and Republicans in the past and said immigration is a top issue for him in deciding which candidate to back.

He said there was no need for either Obama or Congress to take new measures. "It doesn't matter. Enforce the laws that we have here now," he said.

--The Reuters/Ipsos poll of mid-July interviewed 2,014 Americans online. The precision of the Reuters/Ipsos online polls is measured using a credibility interval. In this case, the survey had a credibility interval of plus or minus 2.5 percentage points. (Additional reporting by Rebecca Elliott and Gabriel Debenedetti, editing by Ross Colvin)