President Obama said on Wednesday that he would seek specific authorization from Congress for the military campaign against the Islamic State, opening the door to a lengthy, potentially contentious debate over the nature and extent of American engagement in Iraq and Syria.

Mr. Obama’s announcement, at his post-election news conference, was not wholly unexpected. But it represented a significant shift from his earlier position that while he would welcome congressional backing, he had legal authority to take military action under existing statutes.

Administration officials said Mr. Obama still believed he had that authority, but with the elections over, he concluded that the time was right to petition Congress for more explicit authority.

“The world needs to know we are united behind this effort and that the men and women of our military deserve our clear and unified support,” Mr. Obama said, adding that he would begin a dialogue with congressional leaders when they come to the White House on Friday.

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