Sen. Bernie Sanders (I-Vt.) is debuting an ad on Iowa television on Thursday contrasting his longstanding support for Social Security benefits with President Donald Trump’s comments suggesting openness to cutting them.

The 30-second spot features footage from a CNBC interview last week in which anchor Joe Kernen asked Trump whether “entitlements” ― a Washington term for universal social insurance programs ― would ever be “on his plate.”

“At some point they will be,” the ad shows Trump responding. (A White House spokesman subsequently denied that Trump was signaling openness to benefit cuts.)

The ad then argues that Sanders, who led opposition to possible Social Security cuts during the Obama administration, is the candidate in the Democratic primary best equipped to draw a clear contrast with Trump on an issue like Social Security.

“To defeat Trump, we need a nominee who has always fought to protect Social Security,” the narrator says, as newspaper clips and campaign literature attesting to Sanders’ stances appear onscreen.

The video ends with audio of Sanders himself inveighing against Social Security cuts as photos showing him speaking with seniors appear.

“We are not going to cut the program that millions of seniors and people with disabilities depend upon,” Sanders says in the spot. “If they vote to cut Social Security, they may well not be returning to Washington.”

The video spot goes live in New Hampshire on Saturday as well. The Sanders campaign did not provide details about the scale of the ad buy.

The ability to mount a competitive general election challenge to Trump is a key part of other Democratic candidates’ closing case ahead of Monday’s Iowa caucuses.

But until the Thursday ad, Sanders, the front-runner in an average of public polls in Iowa, had been less explicit than his rivals in making the case for how he would defeat Trump.

Casting himself as the most effective foil for unmasking Trump’s false populism, as he does in Thursday’s ad, is the type of electability argument Sanders has always been comfortable making. From the moment Trump was elected, Sanders sought to remind Trump of the Republican’s campaign-trail promises to preserve Social Security, Medicare and Medicaid ― and to lambaste him for any signs that he might be deviating from those pledges. After the CNBC interview, Sanders declared that Trump “lied” about his commitment to protecting the programs.

The ad is also an implicit dig at former Vice President Joe Biden. Sanders spent much of the past two weeks litigating a fight with Biden that Sanders had instigated over Biden’s past support for cutting Social Security. Among other things, Biden cast a vote in 1995 for a balanced budget amendment to the Constitution that would have threatened Social Security with automatic cuts; embraced the longstanding Washington consensus that entertaining Social Security cuts was a sign of fiscal seriousness; and led 2011 debt-reduction talks with congressional Republicans in which a cut to Social Security’s cost-of-living adjustment was on the table.

Biden now says he would not even consider cutting the program. His plan for “older Americans” proposes increasing Social Security’s revenue enough to make the program solvent and expanding benefits for the most vulnerable groups. And Biden’s campaign is fond of noting Sanders’ praise for the former vice president in 2015, when he recognized Biden’s work on behalf of “the well being of working families and the middle class.”

Still, Sanders insists that Biden’s record provides useful insight into what his priorities would be as president. And with the new ad, he’s casting doubt on Biden’s electability ― a key part of the former vice president’s self-styled appeal ― without explicitly attacking him on-air and risking the kind of mudslinging to which Iowans are famously averse.