Sen. Elizabeth Warren's sweeping "Medicare for All" plan is too much of a political reach to actually be implemented on Capitol Hill, according to Jared Bernstein, who used to advise her Democratic presidential rival Joe Biden.

"A good question for her is, given neither this nor the next Congress will enact your plan, don't tell us what you'd like to do. Tell us about what you would do; what you think is in the realm of possible," Bernstein told CNBC on Monday. "This debate, this is just basically like saying, 'I'm going to buy a unicorn and I'm going to pay for it with a unicorn.'"

In an email follow-up to Monday's "Squawk Box" interview, Bernstein wrote: "My point was that while I very much like the aspirations of the 'Medicare for All' plan, we shouldn't spend too much time arguing about things that are very unlikely to be legislated. That goes for both the plan and its payfors," or how she plans to pay for it.

The Massachusetts senator on Friday announced her Medicare for All proposal would cost the country "just under" the estimated $52 trillion-over-a-decade cost of the current system. It includes $20.5 trillion in new federal spending. Warren also said Friday she would double her wealth tax on billionaires — from 3% to 6% for household net worth over $1 billion — to help pay for it. Her proposed 2% tax on net worth over $50 million remains unchanged.

"In terms of making the numbers added up — yeah, there are a lot of questions there," said Bernstein, who was chief economist for Biden when he was vice president during Barack Obama's presidency.

"My biggest misgiving about the plan is that it's really hard to transition 160 million people from employer-provided coverage — many of them, and I'm one of them, really like that coverage — without a much longer transition plan than I see in here," added Bernstein, who was in the Obama administration when the Affordable Care Act, also known as Obamacare, was being crafted.

While she knows that not every legislator or fellow 2020 presidential candidate will support her Medicare for All plan, Warren wrote, in a Medium post, that her critics should present their own plans.

"Make no mistake — any candidate who opposes my long-term goal of 'Medicare for All' and refuses to answer these questions directly should concede that they have no real strategy for helping the American people address the crushing costs of health care in this country. We need plans, not slogans," she also wrote.

Warren pledged Friday not to raise middle-class taxes to fund her Medicare for All plan, responding to pressure as she emerged as a strong second in the race for the 2020 Democratic presidential nomination.

Also on Friday, Biden, the front-runner in the Democratic primary, said on PBS that the senator is "making it up" when it comes to paying for her Medicare for All plan. Biden told the "NewsHour" program that he can pay for his health-care plan, which firms up Obamacare with a public option at a cost of $750 billion over a decade. "It can be done now. Not in four years. Not in eight years. Not in 10 years."

The Warren campaign did not immediately respond to a request for comment on Bernstein's remarks.