Democratic presidential candidate Bernie Sanders speaks during a recent town hall meeting in Toledo, Iowa. (Photo by Scott Morgan/Reuters)

With less than three weeks remaining before the Iowa caucuses, Democratic hopeful Bernie Sanders still has some major items on his to-do list.

The Vermont senator has pledged both to lay out proposed changes to the country’s income tax rates -- including what top earners would pay -- and deliver a speech on his foreign-policy vision before the Feb. 1 caucuses, when the first votes will be cast in the Democratic nominating process.

On Wednesday, spokesman Michael Briggs acknowledged the commitments but said neither has been firmly scheduled.

Sanders has also come under growing pressure in recent days to release the details of his plan for a shift to a single-payer health-care system, a bold leap and a major policy difference with his Democratic rival Hillary Clinton, who has advocated building on the Affordable Care Act.

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As a senator, Sanders last introduced legislation for a “Medicare-for-all” system in 2013. His campaign has said for months that he will offer an updated plan as a presidential candidate, a point that Briggs reiterated Wednesday.

“We’ve got another proposal that is being worked on,” he said, adding that a release date has not been set.

With Sanders’s recent rise in the polls, the Clinton camp has stepped up its calls for the senator to lay his cards on the table, arguing the former secretary of state has gone further in detailing policy proposals than Sanders.

On Wednesday, for instance, after the Sanders campaign sent out an email to reporters listing how it would pay for several major proposals, Clinton spokesman Brian Fallon took to Twitter to note that there was no mention of a health-care proposal.

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“He left out the $15 trillion proposal that requires across-the-board tax increases on working families? Go figure,” Fallon tweeted.

Later, Fallon added: “If Sanders wants to argue his huge tax hikes on middle class are worth it, fine. But be honest about what is required and let voters decide.”

The 2013 version of Sanders’s “Medicare-for-all” plan did include tax increases, but Sanders aides have argued the plan would save the average family thousands of dollars a year because they would no longer have deductibles or co-payments.

Sanders’s plan for income taxes is also likely to provide fodder for friends and foes alike.

The self-described democratic socialist has said since launching his campaign that he wants corporations and the wealthy to pay their “fair share” of taxes and warned that they will go up substantially. While Sanders has detailed some tax changes, he has not said what what he believes the top marginal income tax rate should be.

Earlier this week, Clinton announced plans aimed at boosting the taxes paid by high earners, including a 4 percent “Fair Share Surcharge” on Americans who make more than $5 million a year.

Sanders, whose campaign has largely focused on economic issues, has also pledged to deliver a major foreign policy speech. Last month, Sanders said he planned to do that in Iowa before the Feb. 1 caucuses.

In some areas, Sanders has put forward very detailed policy, including climate change, immigration and Wall Street regulation.

Anne Gearan contributed to this report.