Democratic and Republican leaders on Wednesday renewed their fitful efforts to impose greater order on a freewheeling presidential race and to bring to heel a pair of political renegades, Senator Bernie Sanders and Donald J. Trump, whose upstart campaigns have roiled the political establishment.

For Democrats impatient to rally around Hillary Clinton as their presumptive nominee, Mr. Sanders has lingered as an obstacle so stubborn and frustrating that only President Obama’s intervention might dislodge him soon.

Mr. Obama is expected to meet with Mr. Sanders in Washington on Thursday and increase the pressure on the irascible Vermonter to defer to Mrs. Clinton. Mr. Obama, White House aides said, intends to nudge Mr. Sanders toward embracing her, stressing that Mr. Sanders can further his policy agenda while unifying the party to defeat Mr. Trump in November.

The White House 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.