Even as his chances of winning the Democratic presidential nomination slip away, Sen. Bernie Sanders and his allies are trying to use his popularity to expand his political influence, setting up an ideological struggle for the soul of the Democratic Party in the post-Obama era.

Aides to Sanders have been pressing party officials for a significant role in drafting the platform for the Democratic convention in July, aiming to lock in strong planks on issues like a $15-an-hour federal minimum wage, breaking up Wall Street banks and banning natural gas “fracking.”

Amid his unexpectedly strong showing in the Democratic primaries, Sanders has tapped his 2-million-person donor list to raise money for liberal congressional candidates in New York, Nevada and Washington state. And in the waning months of Barack Obama’s presidency, Sanders’ allies are testing their muscle against the White House, mounting a public attack on the president’s housing secretary, Julián Castro, over his department’s sales of delinquent mortgages to banks and private equity firms.

“There is a greater goal here,” said Rep. Raúl M. Grijalva of Arizona, a co-chairman of the Congressional Progressive Caucus, who sent a letter to Castro criticizing the mortgage sales. “The contribution of Bernie that will be lasting for us is that we will coalesce around an agenda.”

The pressure from Sanders and his allies is putting the party establishment, which is closely aligned with Hillary Clinton, in a delicate position. Democratic leaders are wary of steering the party too far left, but do not want to alienate the Sanders supporters whose votes Clinton needs in November, or risk losing the vast new donor base Sanders has created.

The institutional bulwarks against Sanders are significant: Hundreds of the party’s “superdelegates” have endorsed Clinton, a signal of her broad support among the party’s power brokers. The Democratic National Committee now relies on Clinton’s fundraising to provide a fifth of its monthly income, an arrangement the Sanders campaign has criticized.

And Clinton is well positioned to block any proposals she would not want to defend in a general election. In January, the party chairwoman, Rep. Debbie Wasserman Schultz of Florida, appointed dozens of Clinton supporters and advisers to the three standing committees of the Democratic Party convention. Of 45 potential members submitted by Sanders, she appointed just three, according to Sanders’ campaign.

Gov. Dannel P. Malloy of Connecticut, a top Clinton surrogate, will be co-chairman of the platform committee. Barney Frank, a former Massachusetts congressman and fierce critic of Sanders and his Wall Street proposals, will be a co-chairman of the rules committee, which governs procedure on the convention floor.

Mark Longabaugh, a senior adviser to Sanders, said he believed the campaign would ultimately be well represented on all the committees as more members are named. But he questioned how Wasserman Schultz had chosen her discretionary appointments.

“Not only are they supporting Clinton, but they have been extremely critical of Bernie Sanders,” 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,” Miranda said.

Some fissures within the Democratic family may be inevitable. For eight years, 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 Sen. Elizabeth Warren of Massachusetts, has become more restive in 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 unauthorized 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 Clinton recently voiced qualified support. But there is a risk, some Democrats said, that Sanders — an independent who is not actually a member of the party — would push the party to embrace positions that could later hurt Clinton and other Democratic candidates.

Sanders is almost certain to win a prime-time speaking slot at the summer convention, providing one of the biggest audiences yet for his views. Some Democrats said they feared a left-wing equivalent of Pat Buchanan’s searing speech at the 1992 Republican convention, when Buchanan, who had failed to win his party’s nomination, called for a “cultural war” against “liberals and radicals.”

Matt Bennett, a founder of the center-left think tank Third Way, said Clinton had so far avoided tacking too far left to compete effectively in a general election. “They need to be careful not to go so far as to hand the Republicans something to beat them over the head with,” Bennett said. “Bans on anything tend to be politically problematic.”