Last year, former vice president Joe Biden debuted a plan to expand Social Security benefits. Under the proposal, people who work for three decades would receive a minimum benefit 125 percent above the poverty line. Those who’ve been receiving benefits for 20 years would receive a higher monthly benefit, as would those who’ve lost a spouse.

Good for Biden! Social Security is popular. So is expanding it. There’s just one problem: Biden’s actually got a history of contemplating cuts to Social Security. As Sen. Bernie Sanders (I-Vt.) pointed out last week, that record would leave Biden uniquely vulnerable to an attack by President Trump come the fall, should he end up as the Democratic nominee, no matter how sincere his change of heart.

The Intercept’s Ryan Grim has published a helpful roundup of Biden’s known remarks and actions on Social Security. In 1984, when the inflation rate was more than 4 percent, Biden called for a one-year moratorium on the program’s annual cost-of-living increase. He’s said he would consider an increase in the retirement age. He was the point person on the Obama administration attempts to reach a deficit deal with Republicans in Congress; Social Security cuts were most definitely part of those conversations.

More recently, there is Biden’s appearance in 2018 at the Brookings Institution, when the former vice president made a muddled statement that seemed to embrace means-testing Social Security. “I don’t know a whole lot of people in the top one-tenth of one percent, or the top one percent who are relying on Social Security when they retire,” he said, before going on to discuss his own tax plans, adding that it (and Medicare) still needs adjustments.” Biden now says he was being sarcastic, and is backed up by PolitiFact, but left-wing commenters remain skeptical of the explanation.

There is no doubt Trump and his supporters will use all this against Biden should he be the nominee. As I am forever reminding people, one of Trump’s main planks in 2016 was his defense of Social Security. It separated him from his GOP rivals and likely was a factor in why seniors supported him at higher rates than they did Hillary Clinton. To be sure, Republicans will attack on Social Security regardless of the nominee: Last year, a press secretary for the Trump reelection effort lied about Democrats making “painful cuts” to Social Security to pay for the Green New Deal and Medicare-for-all. But Biden’s record would let Team Trump recast that argument with at least a patina of fact. And with a decent percentage of his record on tape, if Biden’s the nominee, millions of Americans will see ads with his pronouncements on Social Security over and over again. (Remember, Facebook does not fact-check political ads.)

Yes, it’s almost certainly true that Trump’s rah-rah Social Security is yet another con from a lifetime con man. As far back as the 2016 campaign, his chief policy adviser said Trump was mostly certainly interested in revamping the program. His administration is now proposing making it harder to qualify and continue to receive Social Security disability benefits. But Trump’s a professional entertainer, and his ability to blast through contradictions that trip up lifetime professional politicians shouldn’t be underestimated.

One Democrat who isn’t vulnerable? You guessed it: Sanders. He’s a longtime Social Security champion and last year introduced legislation that would increase benefits for lower-income Americans, as well as change the cost-of-living formula to compensate for the higher inflation seniors experience thanks to their disproportionate medical spending. He said he would pay for it by eliminating the payroll tax cap on income above $250,000 and include dividends and capital gains in that number. The Social Security Administration’s Office of the Chief Actuary says the Sanders initiative would, if adopted, stabilize the program’s finances for next half-century.

It’s tough to overstate how important Social Security is: As the National Institute on Retirement Security pointed out in a study released Tuesday, the program is the only income source for 40 percent of retirees over the age of 60. Age discrimination remains a potent issue. For all the talk of raising the retirement age to account for longer lifespans, a majority of people over the age of 50 are likely to be forced out of their job at some point. This stuff is a day-to-day reality for millions of Americans. Whether or not Biden’s record on Social Security gets a full hearing in the Democratic primary — starting, say, with Tuesday night’s debate — it will certainly get one in a general election if he’s nominated. Don’t say I didn’t warn you.

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