The U.S. Senate will lose one of its most influential — and controversial — conservatives when Jim DeMint resigns in January, as he seeks a national audience for his ideas at the Heritage Foundation. And South Carolina will lose a junior senator who overwhelmingly won re-election in 2010.

The selection of Sen. DeMint to head the nation’s leading conservative think-tank is an indication of just how prominent he has become as an advocate of limited government and fiscal frugality. Indeed, the Greenville Republican emerged in recent years as a national figure in articulating the views of the tea party, a movement that powered the GOP’s capture of the U.S. House in 2010.

Sen. DeMint’s departure won’t add to the Democrats’ margin of power in the Senate, since Republican Gov. Nikki Haley will appoint a replacement to serve until the next general election.

After some initial hesitation, Sen. DeMint decided to abide by his pledge to make this second term his last.

But by running for a second term, he implicitly pledged to complete it, barring disability. That makes his abrupt departure a disservice to those who supported him at the polls in 2010 in the justified expectation that he would serve through 2016.

Mrs. Haley will certainly name a conservative to replace him, though one meeting Sen. DeMint’s level of ideological purity shouldn’t be the standard. The Senate is a political body that demands pragmatic skills.

Sen. DeMint’s habit of letting conservative dogma supersede political common sense has been his weak point, as evidenced by his refusal to support a comparatively minor allocation needed for a federal study that was a prerequisite for deepening Charleston Harbor.

Initially, Sen. DeMint wouldn’t budge on his refusal to help seek the $400,000 allocation because he considered it an earmark. Meanwhile, the essential harbor project was put at risk.

Fortunately, the issue was resolved.

Sen. DeMint’s insistence on ideology over pragmatism also was in evidence as he intervened in other states’ Republican Senate primaries in 2010, stumping for candidates he found more suited to his governing philosophy.

With his help, seemingly long shots running against GOP establishment candidates won primaries in Nevada and Delaware, then lost in the general election.

Some analysts, and some fellow Republicans, faulted Sen. DeMint for what they saw as his role in costing his party good chances to win those seats.

On the other hand, Sen. DeMint also gave timely primary support to Senate candidates Marco Rubio of Florida in 2010 and Ted Cruz of Texas this year. Both won their general election races and are now considered rising GOP stars.

Clearly, Sen. DeMint has the independence of mind to stand outside the party mainstream. That quality should serve him well as head of the Heritage Foundation, which does valuable work on restraining government’s reach and spending in part by supporting research and related op-eds, distributed nationwide. (See Katie Tubbs’ column on our Commentary page today.)

Meanwhile, speculation immediately focuses on Gov. Haley’s selection of our next senator, who will serve through 2014. She said Thursday that she does not want the job herself.

Sen. DeMint reportedly wants her to deem 1st District Rep. Tim Scott as his successor. Now preparing for his second term, Rep. Scott has shown strong conservative credentials. In the next session he will be the House’s only black Republican.

Another familiar name in the rumor mill is former S.C. Attorney General Henry McMaster, who ran against Mrs. Haley in the 2010 GOP primary before supporting her in the runoff.

Former House Speaker David Wilkins of Greenville, who served as U.S. ambassador to Canada, shouldn’t be counted out.

But regardless of whom the governor chooses, let’s hope our next junior senator blends a fiscally conservative ideology with a practical ability to distinguish between wasteful pork and necessary federal spending.