WASHINGTON — Vermont Sen. Bernie Sanders has launched an offensive against former Vice President Joe Biden in the days since last week’s Super Tuesday results essentially rendered the Democratic presidential primary a two-man race.

Sanders’s push, which has included speeches and media appearances, has focused on criticizing aspects of Biden’s record, including past support for the Iraq War, the Wall Street bailout, and cuts to Social Security. The pair have also traded blows on the airwaves, with Sanders launching an ad highlighting Biden’s past calls for freezing Social Security to help balance the budget and Biden responding with a commercial of his own accusing Sanders of engaging in a “false” and “negative attack.” On Sunday, the Sanders campaign sent a memo to surrogates and endorsers urging them to double down on the Social Security critique while highlighting Sanders’s “lifelong fight to defend Social Security.”

“Biden clearly knows this is an enormous weakness in the Democratic primary, and it would be a major political vulnerability in a general election. Bernie does not have that same vulnerability — unlike Biden, Bernie has unwaveringly fought to protect and expand Social Security for his entire career,” reads the memo, which Yahoo News obtained from a campaign source.

Bernie Sanders. (Kamil Krzaczynski/AFP via Getty Images)

The document included a four-page review of moments dating back to 1983 in which Biden expressed openness to cutting Social Security and backed legislation that could have led to cuts. Those quotes highlighted by the Sanders campaign include a 1998 press release from Biden’s Senate office that boasted he was “one of [the] Senate’s most conservative Democrats.” According to the press release, that statement was based on a National Journal analysis of Biden’s 1997 votes, including his support for “banning late-term abortions” and backing a constitutional amendment designed to balance the federal budget.

“Ideological labels have never guided my votes,” Biden said in the press release. “I look at the merits of an issue, how it impacts people in Delaware, how it shapes national policies, and I follow my conscience. National Journal’s voting analysis, however, confirms what I have been saying for the past several years — moderates from both parties are the key political force in the Senate to getting the job done in Congress for the American people.”

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Bernie Sanders and Joe Biden. (Jonathan Ernst/Reuters)

The memo highlighted the fact Biden remained open to raising the retirement age for Social Security benefits in 2007 as he ran for president. The Sanders team’s memo noted reporting from author Bob Woodward that said, while he was vice president, Biden was willing to make cuts to Social Security and Medicare as he sought budget compromises with Republicans. The Sanders campaign memo contrasted all of this with Sanders’s long history of opposing Social Security cuts.

A Sanders campaign aide said the focus on Social Security is aimed at reaching voters between the ages of 40 and 64, who are approaching retirement age. According to the aide, the Sanders campaign sees middle-aged people as a “key swing vote” in a race where Sanders has dominated among younger voters and Biden has won the older vote. Social Security includes a suite of federal programs that provide aid and medical care to the elderly, retired and needy, among others.

The Biden campaign did not respond to a request for comment on this story. However, in the ad responding to Sanders’s Social Security attacks, Biden’s campaign pointed to the fact he is currently calling for expanding Social Security as part of his presidential campaign.

“Joe Biden has always been a strong supporter of Social Security,” a narrator says in the clip.

Biden also responded to another Social Security jab from Sanders last week, tweeting: “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.”

For his part, Sanders has framed his attacks on Biden as a “contrast of ideas” that is focused on policy differences rather than personal attacks. In an appearance on ABC’s “This Week” on Sunday morning, Sanders stressed that he and Biden have vowed to support the other in a general election against Trump even as he reiterated his critique of the former vice president.

“And by the way, I have said if Joe wins, I’ll be there for him to defeat Trump,” Sanders said. “And he has said he will be there for me to defeat Trump.”

Read the Sanders campaign memo below.

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