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Dixie twist: Miss. voter laws lift Cochran to victory

Veteran Republican Sen. Thad Cochran capitalized on Mississippi's unusual election laws to defeat tea party challenger Chris McDaniel and win renomination with the help of Democratic voters.

Cochran and his allies, led by former Mississippi governor and national Republican chairman Haley Barbour, managed the rare feat of sharply increasing voter turnout over totals three weeks earlier in the initial primary voting. The presence of a third candidate in that June 3 balloting kept either of the two leading candidates from reaching the 50 percent needed to win.

Most political observers had expected McDaniel, who led narrowly in June 3 voting, to defeat Cochran in Tuesday's runoff. Outside hard-right conservative groups, from the Club for Growth to the tea party, invested heavily in McDaniel in hopes he would repeat the success of economics professor Dave Brat in knocking off House Majority Leader Eric Cantor of Virginia.

Sen. Thad Cochran speaks to supporters after holding on to his seat after a narrow victory over Chris McDaniel, June 24, 2014 in Jackson, Miss.
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Sen. Thad Cochran speaks to supporters after holding on to his seat after a narrow victory over Chris McDaniel, June 24, 2014 in Jackson, Miss.

But voters do not register by party in Mississippi, where "open primary" rules allow residents to cast ballots in either party's nomination contest. The only voters barred from voting in the Republican runoff were those who had participated in the Democratic primary on June 3.

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That left a vast pool of potential supporters for Cochran, who has been re-elected five times easily since first winning his seat in 1978. His strategist reached out to less motivated Republicans, independents and Democrats—largely African-Americans—in order to expand his vote totals. To pull voters to the polls, they cited the money Cochran had brought home for defense, infrastructure and education programs as a senior member of the Appropriations Committee, and cast doubt on whether McDaniel could do the same or would want to.

Read More Sen. Cochran defeats Tea Party rival in Mississippi Republican runoff

McDaniel, arguing for fresh leadership and spending restraint, managed to increase his vote totals as well, but not enough to win. On Tuesday night, he complained that "liberal Democrats" had decided the outcome, and he declined to concede defeat. He cited unspecified "irregularities" and hinted at the possibility of a legal challenge, though the basis for one remained unclear.

—By CNBC's John Harwood.

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