The delicate issue for Bernie Sanders that his Democratic opponents won't touch

He is also on the verge of tightening control of the race for the Democratic presidential nomination, likely buoyed by strong showings in delegate-rich states like California and Texas that are set to vote on Tuesday. (Sanders leads in both states, according to polls released by CNN on Friday.)

That reality has begun to sink in among the Democratic establishment this week, as it has become much, much clearer in the six days since the Nevada caucuses that this nomination is now the Vermont democratic socialist's to lose. The scrutiny applied to a front-running candidate covers their policy positions, their personality and preparedness for the nation's top job, and yes, even their health.

Consider what President Donald Trump would do with the issue of Sanders' health, given a) that Sanders had a well-documented heart attack last fall while campaigning in Nevada and b) the way in which Trump tried to make Hillary Clinton's health an issue in 2016.

Following an apparent episode of light-headedness at a September 11 memorial, Trump would go after Clinton's health at nearly every campaign rally, "She's supposed to fight all of these different things and she can't make it 15 feet to her car?" he said in October 2016 in Pennsylvania. "Give me a break. give me a break. Give me a break! She's home resting right now. She's getting ready for her next speech, which is going to be about two or three minutes."

Clinton later said she'd had pneumonia, an explanation that did nothing to dampen the attacks from Trump and his allies in conservative and fringe media.

Politics is often a nasty business: playing to peoples' fear, doubts and suspicions rather than their better angels. Trump has taken that inherent nastiness in politics and turned it up to 11. There's seemingly nothing he won't say or do to win -- up to and including trying to raise questions about his opponent's fitness for office, using whatever ammunition is available to him.

And the fact of the matter is, the Clinton and Sanders health situations are not at all the same. Sanders' heart attack and age, while delicate to talk about, are a relevant factor in this race. Every year, Gallup asks voters a series of questions about whether they would be willing to vote for someone with certain attributes. This year, less than 7 in 10 (69%) said they would be willing to vote for a candidate over 70. Age and health make a vice presidential running mate choice more important. And if any future health issues arise, the fact of Sanders' heart attack will absolutely color the way voters look at his fitness for office.

Which brings us back to the difficulty of raising this issue -- and worry -- within the context of the Democratic primary fight. You can't simply come out and say: "What happens if Bernie falls ill in the race against Trump? Has anyone really considered that????" That would likely raise calls of fear-mongering with a political intent. But leaving the conversation for Trump to shape does no favors for Sanders' (and Democrats') chances in the general election.

The closest any of Sanders' rivals have come to trying to force voters to ask that very question (without actually asking it) is former New York City Mayor Mike Bloomberg's campaign, who released detailed information about his heart health on Thursday including his ejection fraction, a number that indicates "how much blood the left ventricle pumps out with each contraction," according to the American Heart Association. (A normal ejection fraction is between 50% and 70%; Bloomberg's was between 60% and 65%). Bloomberg underwent a stent placement in 2000.

"Releasing this single scientific number about heart health could start to put to rest any concerns about Senator Sanders' secrecy about his recent heart attack," Bloomberg spokesman Stu Loeser said. "Mike Bloomberg's doctor shared Mike's number. Will Senator Sanders ask his doctor to do the same?"

The answer, at least at the moment, appears to be no. The Sanders campaign has said repeatedly that it will not release any more information beyond the medical letters it already has made public. That runs counter to Sanders' pledge, in the wake of his heart attack, to release his full medical records. "The people do have a right to know about the health of a senator, somebody who's running for president of the United States," Sanders told CNN last fall. "Full disclosure."

During last week's debate in Las Vegas, the issue of Sanders' broken pledge came up via a question from moderator Hallie Jackson. Here's that exchange:

Jackson: Let's talk about transparency here. Because many Democrats, including most of you on stage, have criticized President Trump for his lack of transparency. But Senator Sanders, when you were here in Las Vegas in October, you were hospitalized with a heart attack. Afterwards you pledged to make quote all your medical records public. You've released three letters from your doctors, but you now say you won't release anything more. What happened to your promise of full transparency?

Sanders: I think we did. Let me tell you what happened. First of all, you're right, and thank you, Las Vegas, for the excellent medical care i got in the hospital two days. And I think the one area maybe that Mayor Bloomberg and I shared, you have two stents as well.

Bloomberg: 25 years ago.

Sanders: Well, we both have two stents, it's a procedure that is done about a million times a year. So we released the full report of that heart attack. Second of all, we released the full -- my whole 29 years in the Capitol, the attending physician, all of my history, medical history. And furthermore, we released reports from two leading Vermont cardiologists who described my situation and, by the way, who said Bernie Sanders is more than able to deal with the stress and the vigor of being president of the United States. Hey, follow me around the campaign, three, four, five events a day, see how you're doing compared to me."

The conversation then turned to former South Bend, Indiana, Mayor Pete Buttigieg, who is by far the youngest candidate in the Democratic race at 38. Buttigieg said he didn't think Sanders' response was transparent, and he added:

"Transparency matters. Especially living through the Trump era. Now under President Obama the standard was that the president would release full medical records. Do a physical and release the readout. I think that's the standard that we should hold ourselves to as well. Now President Trump lowered that standard. He said just a letter from a doctor is enough. A lot of folks on this stage are now saying that's enough. But I am certainly prepared to get a physical, put out the results. I think everybody here should be willing to do the same. But I'm actually less concerned about the lack of transparency on Sanders' personal health than I am about the lack of transparency on how to pay for his health care plan."

Now, remember that Trump himself has broken with past presidential precedent by refusing to release any sort of detailed medical records. A letter released during the 2016 campaign by his personal physician and claiming that the billionaire businessman would be the healthiest person ever to be president if elected has come under scrutiny since the physician has said Trump dictated its content to him. Trump, who has a common form of heart disease and is clinically obese, also made an unscheduled trip to Walter Reed hospital in November for what the White House called a "quick exam and labs" as the first stage of his annual physical. But that trip did not follow the protocol of a routine presidential medical exam, a person familiar with the matter told CNN. "For any man of that age and medical history," CNN's Dr. Sanjay Gupta wrote at the time "an unexpected visit to the hospital is concerning."

And Buttigieg hasn't released medical records either, though he said in the debate that he was "certainly prepared" get a physical and release the results.

But Buttigieg's debate response speaks to how difficult it is to make Sanders' heart health (and overall health) an issue. Buttigieg, sensing danger, found a way to transition quickly from Sanders' alleged lack of transparency regarding his health to a lack of transparency on how Sanders would fund "Medicare for All."

Which, candidly, lets Sanders off the hook.

The question is whether anyone aside from Bloomberg will keep up the drumbeat regarding concerns over Sanders' health -- in both the short term and the long term. (Because Trump surely will, not to mention his supporters and/or supportive SuperPACs.)

It is, without question, a decidedly delicate subject. But also one that could be the difference between beating Trump in the fall or losing to him.