After two nights of convention speeches focused on Mrs. Clinton’s virtues and attempts to make peace with Sanders supporters, Clinton campaign officials sought to address the threat of radical Islamists — an omission early on that Republicans had criticized. And in a shift from only about a decade ago when they largely avoided the issue, Democrats used much of Wednesday to advocate gun control, sending relatives of those murdered in Newtown, Conn., and Charleston, S.C., as well as a former congresswoman, Gabrielle Giffords, to recount their stories.

But virtually all of the anticipation on Wednesday surrounded Mr. Obama and the symbolic passing of the torch to Mrs. Clinton after she became the party’s nominee on Tuesday night.

Mr. Obama’s resounding endorsement of his one-time rival was the final consummation of a political alliance over a decade in the making, since Mrs. Clinton flew to Chicago in 2004 to raise money for a 42-year-old state senator and discovered a phenom.

Back then he was the one who benefited from the imprimatur of a well-established political star, and her support continued to prove critical over the years. After he won the presidential nomination that she expected to be hers in 2008, Mrs. Clinton put aside her resentment and helped him unify a divided Democratic Party. And later that year, she again came to his aid by agreeing to become his first secretary of state.

Mr. Obama is the one riding high now, his approval rating over 50 percent. And his image is only enhanced as voters view him, in his final months as president, through the prism of a race to replace him that features two deeply unpopular candidates.