“Not only are they supporting Clinton, but they have been extremely critical of Bernie Sanders,” Mr. Longabaugh said. “That doesn’t seem like the right way to go if we want to have a convention that is evenhanded.”

Luis Miranda, a spokesman for the Democratic National Convention, said the party was “committed to an open, inclusive and representative process” to draft the platform.

“Both of our campaigns will be represented on the drafting committee,” Mr. Miranda said.

Some fissures within the Democratic family may be inevitable. For eight years, Mr. Obama’s presidency has muted ideological disagreement within the party. His moral authority as the country’s first black commander in chief, his popularity with grass-roots Democratic voters and his political battles with Congress have worked to squelch the kind of ideological battles that have divided the Republican Party.

But the Democrats’ liberal wing, including lawmakers like Senator Elizabeth Warren of Massachusetts, has become more restive in Mr. Obama’s second term. Liberal groups have pressured the president on recent appointments to the Treasury Department and his deportation policies. At the same time, a younger generation of activists is challenging presidential candidates in both parties on issues of criminal justice, police violence and the rights of undocumented immigrants.

“A boldly populist, people-oriented type of platform is massively appealing to those who have come of age during the financial meltdown and the period afterward,” said Kurt Walters, the campaign director at Rootstrikers, a group that favors limiting the influence of big donors in politics.

The party is relatively unified on raising the minimum wage to $15, and Mrs. Clinton recently voiced qualified support. But there is a risk, some Democrats said, that Mr. Sanders — an independent who is not actually a member of the party — would push the party to embrace positions that could later hurt Mrs. Clinton and other Democratic candidates.