She embraced that label and slammed Biden as "naive" for thinking he could work with Republicans as president. She warned Democrats against picking a "Washington insider" and pointedly refused to endorse Biden until weeks after exiting the race.

Now, those bitter primary clashes are a distant memory.

Warren, a Massachusetts senator and leading progressive, has become an unlikely confidant and adviser to Biden, the presumptive Democratic presidential nominee. They talk every 10 days or so, according to aides to both politicians who spoke on condition of anonymity to freely describe their relationship. Those forums have provided opportunities for Warren to make a case on top policy issues to Biden, who ran a more centrist primary campaign.