WASHINGTON — Mayor Giuliani’s bid to win over social conservatives got a huge boost this morning with the endorsement of leading evangelist Pat Robertson.

The founder of the Christian Broadcasting Network and a longtime leader of the Christian right, Mr. Robertson announced his support at a Washington press conference, citing Mr. Giuliani’s stewardship of New York City and his strong stance against terrorism.

“To me, the overriding issue before the American people is the defense of our population from the bloodlust of Islamic terrorists,” Mr. Robertson said.

RELATED: See video of Robertson’s endorsement

His backing comes as Senator Brownback of Kansas, who dropped out of the Republican presidential race last month, threw his support today behind Senator McCain. The moves underscore the deep split in support among prominent social conservatives for the top Republicans.

Earlier this week, another leading member of the evangelical movement, Paul Weyrich, a co-founder of the Moral Majority, endorsed Mitt Romney, spurring talk that Christians were about to coalesce around his candidacy. With Mr. Robertson’s endorsement of Mr. Giuliani, that speculation will likely dim.

The candidate hurt most by the flurry of social conservative endorsements is Fred Thompson, who at one time appeared on the cusp of becoming the consensus choice for Republicans disenchanted by Messrs. Giuliani, McCain, and Romney. But his campaign has struggled to generate excitement.

Mr. Giuliani, whose support of abortion and gay rights has been well documented, spoke at Mr. Robertson’s Regent University in June, and the two have spoken about sharing a personal bond as survivors of prostate cancer.

The former mayor has tried to cast himself as the Republican with the best chance at defeating the Democratic front-runner, Senator Clinton, but Mr. Robertson said his choice was “not a calculated decision” about electability. Rather, he said, it was about security. “America must be kept safe,” Mr. Robertson said.