The rivalry between Joe Biden and Bernie Sanders is deepening as the Democratic presidential candidates are increasingly at loggerheads over their support of Social Security, signaling a battle ahead that could last well beyond the Iowa caucuses.

Biden’s campaign released a video late Tuesday accusing Sanders of “dishonest” attacks, while Sanders’ official Twitter account countered: “Let’s be honest, Joe. One of us fought for decades to cut Social Security, and one of us didn’t.”

Sanders, a Vermont senator, has accused Biden of advocating for budget deals as a senator from Delaware that would have curtailed entitlement spending over time. During his decades on Capitol Hill, Biden supported a balanced budget constitutional amendment, but he says he would not have “cut” Social Security benefits as part of the deals. In his current campaign, Biden proposes expanding Social Security benefits and raising more revenue by lifting the cap on the income that is subject to Social Security payroll taxes.

Biden tacitly acknowledged on Wednesday that he’s moved to the left over the years on Social Security. But he compared that with Sanders’ legislative history on gun control, an issue that — like Social Security — is a core concern to many Democratic voters.

Sanders “voted to protect gun manufacturers,” Biden said in an MSNBC interview, referring to Sanders’ vote to give weapons makers immunity from civil liability. Hillary Clinton skewered Sanders over that position during their 2016 primary battle.

“He’s indicated that was past,” Biden said, suggesting Sanders has shifted positions on guns in a way that’s more acceptable to Democratic primary voters.

The sniping comes at a tumultuous moment in the Democratic primary as candidates are increasingly feuding with one another on multiple fronts. Tensions within the party soared on Tuesday after The Hollywood Reporter published an interview in which Clinton ripped into Sanders, reigniting divisions from the 2016 campaign and raising concerns that Democrats may struggle to unify behind a nominee and defeat President Donald Trump.

It seemed for a period on Wednesday that the tensions between Biden and Sanders may cool. As a current senator, Sanders is stuck in Washington to sit as a juror in Trump’s impeachment trial, sidelining him from the campaign trail. Biden, meanwhile, spoke of the need for Democrats and the country to come together.

“We not only have to unite our party, we have to unite our country,” he told voters in Mason City, Iowa.

During his interview with MSNBC earlier on Wednesday, Biden demurred when asked whether Sanders had “lied” when he and his campaign aides asserted Biden sought to cut Social Security benefits. Biden also noted that Sanders apologized to him after one of the Vermont senator’s top supporters wrote that the former vice president has a “corruption” problem.

“Sometimes campaign staff gets a little ahead of the candidate,” Biden told the network, later adding, “I accept his apology, and I hope we can argue on the facts.”

But hours later, Biden pointedly noted to reporters that Sanders’ apology was only about a high-profile supporter suggesting the former vice president is corrupt. The senator, Biden said, is still coming after him on Social Security.

The former vice president also had tough words about the Senate’s impeachment trial of Trump.

“I was embarrassed for the institution,” Biden told MSNBC when asked about Trump’s lawyers stating clear mistruths during the proceedings, which are expected to end with Trump’s acquittal on charges that he abused his power and obstructed Congress.

The impeachment case is pegged to Trump pressuring Ukrainian officials to launch an investigation into Biden and his son Hunter Biden, based on discredited theories about the younger Biden’s business dealings in Ukraine while his father handled U.S. foreign policy in the country as vice president.

The elder Biden on Wednesday dismissed the possibility that the Senate could call him or his son to testify. Asked by reporters to respond to reports that Democratic senators are involved in such talks, he replied multiple times, “No, they’re not.” He ignored other questions on the matter.

In Washington, Senate Minority Leader Chuck Schumer agreed with Biden’s assessment. “That’s not even on the table,” he said.

The scenario nonetheless surfaced Wednesday in Osaga, Iowa, where a voter asked Biden whether he’d consider testifying as a way to call Trump and the GOP’s bluff.

“The reason why I would not make the deal: This is a constitutional issue, and we are not going to turn it into a farce, into a political theater,” Biden said, later adding of Trump, “I’m not going to play his game. It’s the Senate’s job now to try him. My job now is to beat him.”

(AP)