UPDATE 3-Conservative US Senator DeMint to resign, head up think tank

* DeMint has been a leading voice in conservative movement

* Criticized fellow Republicans in House on taxes

* Move not likely to alter balance of power in Senate

WASHINGTON, Dec 6 (Reuters) - U.S. Senator Jim DeMint, a

leading voice in the modern American conservative movement who

has riled fellow Republicans as well as Democrats, surprised

both parties on Thursday by declaring he was retiring from the

chamber to head a conservative think tank.

DeMint's announcement came two days after he won praise from

hard-liners by ripping into the top Republican, Speaker of the

House of Representatives John Boehner.

The 61-year-old South Carolinian, who preaches the value of

small government and freely criticizes fellow Republicans,

blasted Boehner for violating a long-time party taboo: proposing

increased revenue as part of a deficit-reduction plan.

"This isn't rocket science," DeMint said. "Everyone knows

that when you take money out of the economy (with tax hikes) it

destroys jobs."

DeMint's departure in January is not expected to affect the

balance of power in the Senate, which Democrats are set to

control next year, 53-47.

South Carolina Governor Nikki Haley appears certain to

appoint a fellow Republican to replace DeMint through 2014, when

a special election will be held.

A senior Republican speculated that Haley, seen as a rising

star, will choose either Tim Scott or Michael Mulvaney, who

represent South Carolina in the House of Representatives, and

perhaps run for the Senate seat herself in 2014.

DeMint was first elected to the Senate in 2004 after six

years in the House of Representatives.

In many ways, he has seemed ahead of his time in the Senate

with his small-government views, which became more popular in

recent years with the rise of the anti-Washington Tea Party

movement.

That movement suffered some big losses in last month's

election, however, when President Barack Obama won a second term

and DeMint's Republicans failed to win back the Senate.

Last year, DeMint was a founder of the Senate Tea Party

Caucus, which now has four members. Three more Tea Party

favorites were elected to the Senate.

With his uncompromising style, DeMint has often

single-handedly tied the Senate in knots by raising objections

to a host of measures to increase the scope of government.

Resulting gridlock fanned by DeMint and others on both sides

of the political aisle contributed to record low approval

ratings for Congress at or near single digits.

PARTISAN BICKERING

"I'm leaving the Senate now, but I'm not leaving the fight,"

DeMint said in a statement. "I've decided to join The Heritage

Foundation at a time when the conservative movement needs strong

leadership in the battle of ideas."

Founded in 1973, the Heritage Foundation, a few blocks from

the U.S. Capitol, is one of the nation's leading think tanks.

With 700,000 members, it has helped shape the debate in

Washington for years.

The Senate has long been known as "the world's most

deliberative body." But in recent years, the gridlock has made

it more frustrating and less attractive.

Earlier this year, Senator Olympia Snowe of Maine, a leading

moderate, announced her retirement, fed up with the partisan

wrangling.

Darrell West of the Brookings Institution, a liberal think

tank, described DeMint's career move as highly unusual, yet

potentially smart.

"It is surprising that a major political leader believes he

can get more done working at a think tank than being a member of

the U.S. Senate," West said.

"DeMint seems to believe that focusing on long-term ideas

outside elective office has greater potential than passing

legislation. It is an extraordinary conclusion in many respects,

but he may well be right," West said.

Ned Ryun, head of Tea Party group American Majority Action,

said he hoped DeMint would move Heritage Foundation further to

the right, criticizing it for proposing years ago an individual

mandate for health insurance, an idea Democratic President

Barack Obama included in his healthcare overall.

"Heritage has a robust presence and the resources to be a

very prominent platform," Ryun said. "Would I love to have him

in the Senate? Absolutely. But he said he was going to retire

anyway."

Republican Senator Lindsey Graham said DeMint, a fellow

South Carolinian, will be missed.

"What he's done over the last four years to build a

conservative movement, to get people involved in politics ... is

going to be part of a great legacy," Graham said.

"Jim made the Republican Party, quite frankly, look inward

and do some self-evaluation," Graham said. "Conservatism is an

asset, not a liability."

Senate Majority Leader Harry Reid, a Democrat who has often

tangled with DeMint, told reporters: "I've always liked the guy

- even though I disagree with so much of what he's done."

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