After years in which Donald Trump assured the public that he'd never cut Social Security, Medicare, or Medicaid, the president adopted a different posture this week. Asked by CNBC's Joe Kernen whether "entitlements" would ever end up on his plate, Trump replied, "At some point they will be.... And at the right time, we will take a look at that."

When Kernen followed up, asking about Trump's willingness to "do some of the things that you said you wouldn't do in the past," the Republican added, "We're going to look."

It was the election-year message Democrats were eager to hear. Senate Minority Leader Chuck Schumer (D-N.Y.), who's been relentlessly on-message this week, shifted his focus a bit yesterday, telling reporters at the start of a Capitol Hill press conference, "Even as the impeachment trial is underway, Trump is still talking about cutting your Social Security."

Get the msnbc newsletter. This site is protected by recaptcha

This did not go unnoticed at the White House.

On Thursday, the president tried to clean up his own mess. "Democrats are going to destroy your Social Security," Mr. Trump tweeted shortly before leaving the White House for a campaign-related event in Florida. "I have totally left it alone, as promised, and will save it!"

It's worth unpacking this, because the issue is likely to be one of the dominant focal points of the presidential election.

First, the idea that Democrats -- who created Social Security and have spent the better part of a century championing it -- are "going to destroy" the social-insurance program is plainly ridiculous.

Second, the president may want people to believe he's "totally left it alone," but to the degree that reality matters, his White House budget plans have proposed tens of billions of dollars in cuts to Social Security. Those proposals were ignored by lawmakers, but there's an obvious discrepancy between trying to slash a program and leaving it alone.

Also note Trump's vow to "save" Social Security. Voters are likely to hear this again in the coming months, because the program's opponents have used this and similar rhetoric for years as a euphemism for "cutting" Social Security. The idea is, by cutting benefits, "reform" advocates would extend Social Security's future -- thereby "saving" the program.

The flip side, of course, is that "reform" advocates could also bolster Social Security's long-term finances by tweaking payroll taxes and generating additional revenue, rather than cutting benefits.

But as important as these details are, the bigger picture remains the same: one of the key 2020 fights will be over the future of Social Security, and it appears to slowly be dawning on Trump that he'll be fighting this battle from a position of political weakness.

MORE: Today's Maddowblog