President Obama will use an Oval Office meeting with Senator Bernie Sanders of Vermont on Thursday morning to delicately nudge the losing Democratic presidential hopeful toward a full embrace of Hillary Clinton’s candidacy and a unified party effort to defeat Donald J. Trump in the fall, according to administration aides.

The conversation, the fifth that Mr. Obama will have had with Mr. Sanders since the primaries began, is to be part of a choreographed series of moves Mr. Obama set in motion this week that are designed to quickly bridge the divide between the two Democratic presidential candidates laid bare in the last few months. The strategy will culminate with the president’s formal endorsement of Mrs. Clinton in the coming days, followed by an appearance with her on the campaign trail soon after.

Mr. Obama’s message is intended to be unmistakable: The time has come for Mr. Sanders to harness his formidable constituency in support of his onetime rival even as he continues to press for the progressive policies that animated his base. But it is also clear to the White House that Mr. Obama must broker this particular intraparty peace treaty with patience and respect, or risk angering Mr. Sanders and his millions of supporters.

To that end, the president will delay any endorsement of Mrs. Clinton until after the meeting with Mr. Sanders. And a joint appearance is unlikely until after the Democratic primary in the District of Columbia on Tuesday. Mr. Obama will not pressure Mr. Sanders to make a concession before that contest, the final one in the long primary season, advisers said.