As the Democratic primary entered its final phase this week, leaving Joe Biden and Bernie Sanders vying for the nomination, the two sparred Thursday night over Social Security, trading barbs on Twitter and setting up the contours of the remaining months of the campaign.

This isn’t the first time the former vice president and the senator from Vermont have gone at it over this issue — in January, they feuded over Biden’s record on the entitlement program. But the stakes are higher now that it’s a two-man race. (Rep. Tulsi Gabbard is still in the race, but she has not amassed enough support to be considered a serious contender.)

The late-night back-and-forth began with Biden commenting on a video of President Trump at a town hall hosted by Fox News where the president says he will cut social safety net programs in his next term.

“Here’s the deal, folks: social security is on the ballot this year, and the choice couldn’t be clearer: I’ll protect and expand it. Donald Trump will cut it and take it away,” Biden tweeted.

Sanders chimed in shortly thereafter.

“Here’s the deal: Joe Biden has repeatedly advocated for cuts to Social Security. I’ve fought my whole career to protect and expand it,” he wrote.

“Get real, Bernie. The only person who’s going to cut Social Security if he’s elected is Donald Trump. Maybe you should spend your time attacking him,” Biden shot back.

“Get real, Joe. One of us has a history of not only fighting cuts to Social Security but working to expand benefits. And that’s why we are the campaign best positioned to defeat Donald Trump,” Sanders responded.

Sanders and his supporters have criticized Biden over past comments on Social Security — an issue of utmost importance to the most reliable voting bloc: those over 65 years old — highlighting a video that has been deemed misleading by some fact-checkers. In the video from 2018, Biden is seen commending former House speaker Paul D. Ryan (R-Wis.) for his proposed cuts to Social Security. But Biden says he was being facetious, and later in the speech he talks about the need to keep Social Security intact.

There were times during Biden’s Senate career when he supported policies to lower the deficit or balance the budget that could have frozen spending on Social Security.

But his aides point to many occasions when Biden voted to preserve Social Security. He was also a staunch defender of the program as vice president. Though, one year President Barack Obama proposed using a different calculation for cost-of-living increases in Social Security that would have resulted in cuts, which Sanders successfully fought to block.

Sanders told reporters Wednesday that he did not want to run a campaign of personal insults but would focus his attacks on Biden on the issues. He rereleased a TV ad he ran in January that hits at Biden on Social Security, which contains audio of Biden as a senator in 1995 discussing freezing spending for entitlement programs.

“Well, we’ve got some bad news for them,” Sanders says in the spot. “We are not going to cut Social Security. We’re going to expand benefits.”