WASHINGTON—Newt Gingrich won the Republican primary in South Carolina by a wide margin, throwing open the race for the party’s presidential candidacy.

Gingrich, the former U.S. House of Representatives speaker, defeated Mitt Romney, long believed to be the front-runner, 40.4 percent to 27.9 percent, according to counts after polls closed Saturday.

Over the last week, Gingrich’s poll numbers climbed in the conservative state, seen as the last chance to keep Romney, the former Massachusetts governor and a relative moderate, from winning the nomination.

Gingrich, who had performed poorly in Iowa and New Hampshire, was given an 11th-hour boost when longtime backer Sheldon Adelson gave $5 million to an unaffiliated pro-Gingrich political action committee.

The committee, Winning America’s Future, blasted the state with ads associating Romney’s past as a venture capitalist with job loss. South Carolina suffers from high unemployment.

Gingrich, a one-time Georgia congressman, also performed well in the CNN debate in the state on Thursday night, scoring ovations with his attacks on the media for its focus on his two earlier failed marriages.

Adelson, a major backer of Jewish and pro-Israel causes, had told associates that he was ready to donate another $5 million to Gingrich’s cause if he performed well in South Carolina.

The next primary state is Florida on Jan. 31, where pro-Israel messaging is likely to factor in a state with a substantial Jewish community.

Rick Santorum, the former Pennsylvania senator who scored a virtual tie with Romney in Iowa and who also casts himself as the conservative to beat Romney, scored 17 percent in South Carolina.

Rep. Ron Paul (R-Texas), who is running a campaign emphasizing small government, cutting foreign aid including to Israel and reducing the U.S. military profile overseas, scored 13 percent.