Bernie Sanders’ ballyhooed speech on socialism is now on indefinite hold.

Details about how Sanders would pay for his proposed single-payer national health insurance program to provide Medicare for all Americans have yet to be fleshed out -- even though a July 30 post on his campaign website says the Vermont senator would file legislation on single-payer “perhaps as soon as next week.”

And with two-and-a-half months to go before the first votes of 2016 are cast, the candidate whose raison d’etre is ending income inequality has yet to unveil any details of his tax plan, such as whose tax rates would go up and by how much.

As Sanders attempts to evolve from a niche politician who appeals to a frustratingly uniform group of young, white progressive voters to a major threat to the Hillary Clinton presidential juggernaut, he appears to be stalled on the threshold of mainstream success. Once sure-footed and decisive in expressing his view of the most important issue facing the country, income inequality, Sanders now appears to be holding back, hesitant to put meat on the bones of his big-picture ideas.

The socialism speech – billed by his campaign as a major address where Sanders, a self-described Democratic socialist, would finally explain his definition of a politically loaded term that is deeply personal to him – provides perhaps the most concrete example.

Sanders first told reporters that he was planning to deliver a “major speech” outlining his political philosophy in October. “I think we have some explaining to do,” he said, admitting the tag of Democratic socialism wrongly makes some people “very, very nervous.”

“What we’re probably going to do to begin with is hold a major speech in the not-too-distant future, to define exactly what I mean by Democratic socialist,” Sanders said during a campaign stop in Iowa. “To me, Democratic socialism means democracy. It means creating a government that represents all of us, not just the wealthiest people in the country.”

But it’s been postponed indefinitely because Sanders has not yet finished writing the high-risk speech, or agreed that he is ready to deliver it. The delay highlights another issue facing the Sanders camp – an apparent disconnect between the candidate and his staffers, who say they are pushing the senator to give his speech sooner rather than later.

“I personally think we’re ready, he’s just got to agree we’re ready,” said senior strategist Tad Devine. “It’s very much on the horizon, it’s just a question of timing. He wants to finish the speech first.”

Sanders still likes to write all of his speeches himself – even when an aide offers a draft, he will typically rework the specific language. The mere prospect of the speech drew comparisons to other Democratic presidential candidates who confronted sensitive questions about faith or belief – putting it in line with Barack Obama's 2008 speech in Philadelphia about race, and John F. Kennedy's speech about his Roman Catholic faith in 1960. And Sanders camp said it was not backing away.

“We want to do this,” Devine said. “It’s not because we don’t want to talk about being a socialist. No way. Just the opposite. This is an opportunity for him to explain how his political philosophy fits squarely into the Democratic Party.”

It’s also not clear when Sanders will provide details of how he plans to pay for his health insurance plan. “Like with the socialism speech, there’s no set date,” Devine said of the single-payer program, as well as the related tax policy details. “I would admit there’s delay. I would attribute that to a desire for it to be a fully costed analysis. We’re not trying to be evasive. He’s trying to be serious.”

During the second Democratic debate Saturday night, Sanders said he was not ready to say how much he would raise the country’s top marginal income tax rate. “We haven’t come up with an exact number yet,” he told CBS news correspondent Nancy Cordes. “But it will not be as high as the number under Dwight D. Eisenhower, which was 90 percent.”

He wiggled out of providing specificity with humor. “I’m not that much of a socialist compared to Eisenhower,” he joked, landing his first applause line of the night.

In the past, Sanders has introduced single-payer legislation in Congress that included a 2.2 percent income tax across the board, as well as a 6.7 percent payroll tax for employers. While payroll taxes are split between the employer and the employee, economists and the Congressional Budget Office have said that most of those fees are carried by workers in the form of lower wages. Sensing opportunity, the Clinton camp went on offense against Sanders on Tuesday, hoping to cast an unflattering light on him for supporting legislation that would raise taxes on the middle class, while Clinton has promised that none of her proposals involve a middle class tax hike.

"Bernie Sanders has called for a roughly 9-percent tax hike on middle-class families just to cover his health care plan," Clinton spokesman Brian Fallon said, referring to his 2013 single-payer legislation, "and simple math dictates he'll need to tax workers even more to pay for the rest of his at least $18-20 trillion agenda."

In an interview, Sanders’ campaign appeared to be backing away from the senator's own previous bills, aware that any possible tax hike that could be a deal breaker for middle class voters. Devine said “what Bernie wants to do now [in terms of his healthcare proposal] is not exactly like the one proposed in past legislation.”

Any proposal the campaign will unveil, said Devine, is unlikely to involve tax rate increases on the middle class. “I don’t really see that,” he said. “I don’t see tax rate increases involving anyone other than people making a lot of money.”

Devine also pushed back against the idea that the Sanders campaign’s delays in rolling out specifics – on his tax plan or anything else – reflected a calculated consideration of the political realities of the race and his approach.

“That is a not a back of the envelope endeavor,” Devine said of the tax plan. “I don’t know if the Congressional Budget Office itself will be involved, but it’s going to be legit. The think tanks are going to say these are real numbers. This is not somebody’s tax plan that makes no sense in the real world. He’s a serious legislator.”