Donald Trump seems entirely immune to the things that normally destroy most politicians. He can lie and get away with it. He can say offensive things and claim he refuses to be politically correct. He can even brag about assaulting women and then assert the entire matter was years ago, so no biggie.

So the question has been raised and must be considered: Does this man have a weakness? Yes he does, and Democrats are preparing to use it as kryptonite against they seemingly invulnerable President-elect.

What is it? One word for you: Medicare.

Today, Bernie Sanders, surrounded by Chuck Schumer and Nancy Pelosi, took Trump’s own words and used them to attack House Speaker Paul Ryan, who has floated the idea of turning Medicare into a voucher program. Sanders said:

“Let me begin with a quote made by a Republican candidate for president ― a candidate who reached out to working people and who reached out to senior citizens ―and during the campaign this is what that Republican candidate said. He said: ‘I am not going to cut Social Security like every other Republican. And I am not going cut Medicare or Medicaid. Every other Republican is going to cut them’ ― end of quote. Mr. Trump, we are going to hold you accountable!”

For years, Ryan has enthusiastically endorsed plans to change Medicare from a government-provided insurance plan to a flat voucher with which seniors would have to purchase their own insurance. Many health care experts warn that a voucher plan would endanger traditional Medicare as private insurers lure healthier and more affluent seniors away. And any voucher would almost certainly fail to keep pace with the cost of health care.

Chuck Schumer echoed Sanders and said the Democrats are ready to protect senior citizens:

“Republicans here in Washington are gearing up for a war on seniors ― and they shall not, they must not, prevail.  Democrats will not let them win this fight. I say to my Republican colleagues: Turn back, because we will fight you on this tooth and nail. We will win, you will lose.”

So if Trump and his colleagues decide to start slashing the social safety net, they will be met with opposition from a Democratic Party looking for an issue to use as a cudgel in the 2018 midterm election. And if Trump were to agree to a compromise on the issue, he would be opening himself up to charges that he’s just another one of the politicians he railed against on the campaign trail. But if he does nothing, he risks being painted as weak an ineffective by those on the right.

Does Trump have a fatal weakness? We should find out very soon.