That dynamic—Biden responding to or ramping up criticism of Warren, while oftentimes going out of his way to praise Sanders’ honesty—has been escalating in recent weeks. With less than three months until the Iowa caucuses, the former vice president has sharpened his messaging against the Massachusetts senator over substance and style, and has largely left Sanders, who also remains a top-tier contender for the Democratic nomination, alone.

While the two politicians in their late seventies voted in opposite directions on a variety of high-stakes decisions (including to authorize the Iraq War and NAFTA, which Biden was for and Sanders was against), they have enjoyed a generally collegial Senate relationship. Sanders often refers to Biden as his “good friend” whom he disagrees with on many issues. And Biden tends to do the same.

“I know genuinely the senator does have a respect for Vice President Biden,” a senior Sanders 2020 campaign adviser told The Daily Beast. “They’ll joust on the debate stage, that’s as far as it will go.”

In contrast, his relationship with Warren, dating back at least 17 years, is illustrated in vivid detail during a 2005 Senate Judiciary Committee hearing over a bankruptcy bill that Biden supported and Warren was adamantly against.