By Jeff Mason

WASHINGTON (Reuters) - Thirty-one percent of U.S. Republicans favor a new nuclear deal with Iran, creating a challenge for their party's lawmakers who largely oppose the framework accord sealed between Tehran and world powers, a Reuters/Ipsos poll showed on Wednesday.

Another 30 percent of Republicans oppose the pact, while 40 percent are not sure, according to the poll, which revealed a sharp split in the party as its leaders ramp up opposition to the deal championed by President Barack Obama, a Democrat.

The White House has launched a broad effort to persuade U.S. lawmakers and other critics to embrace the framework agreement reached last week between Iran, the United States, Britain, France, Germany, Russia and China.

Many details remain to be worked out for a final deal to be completed by the end of June. Public support will be critical to the White House effort to sway skeptical members of the Republican-controlled Congress, many of whom see the deal as a dangerous concession to a country that sponsors terrorism.

The nuclear deal foresees lifting U.S. and international economic sanctions on Iran in exchange for compliance with restrictions on its nuclear program.

The poll showed Obama has some selling to do among members of his own party. Though 50 percent of Democrats supported it, 10 percent were opposed and 39 percent were not sure.

Among independents - an important constituency group for both parties ahead of the 2016 presidential election - 33 percent voiced support, 21 percent registered opposition, and 45 percent said they were unsure about the deal.

Americans overall are mixed on the deal. Of those polled, 36 percent were in favor, 18 percent were against, and 46 percent unsure.

The Reuters/Ipsos online poll surveyed 2,291 American adults between April 3 and April 7, after the Iran deal was announced. Those polled included 893 Democrats, 803 Republicans and 320 independents.

People from both parties diverged substantially in their views of a U.S. rapprochement with Iran.

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Sixty percent of Republicans said the United States should hold a hard line with its longtime foe and maintain or expand current sanctions, compared with 23 percent of Democrats who said the same.

Meanwhile, 48 percent of Democrats said Washington was right to improve diplomatic relations with Tehran, compared with 18 percent of Republicans.

The poll showed little support among members of both parties for using military force as a sole method for preventing Iran from developing a nuclear bomb. Five percent of Democrats supported such an option, along with 11 percent of Republicans and 6 percent of independents.

Support for the combined use of diplomatic channels along with military force was higher, however. Fifty percent of Republicans favored that combination, along with 35 percent of Democrats and 42 percent of independents.

Americans are concerned that the framework deal would make Iran a greater threat to Israel, the poll showed.

Forty-five percent of respondents said Iran would be a greater threat to the U.S. ally as a result of the deal, compared with 29 percent who said it would become a lesser threat and 26 percent who said it would have no impact.

Broken down among parties, 63 percent of Republicans believed Iran will become a greater threat to Israel as a result of the deal, while 36 percent of Democrats believed the same.

The Reuters/Ipsos poll is measured with a credibility interval. It has a credibility interval of plus or minus 2.3 percentage points for all adults polled, 3.9 percentage points for Republicans, 3.7 percentage points for Democrats, and 6.2 percentage points for independents.

For poll click here http://bit.ly/1N9nYUG

(Reporting by Jeff Mason; Editing by Leslie Adler and Jonathan Oatis)