Susan Rice is among the contenders as a running mate for Democratic presidential nominee Joe Biden. Rice worked closely with Biden in the Obama administration and regularly briefed him on pressing foreign policy matters when she served as national security adviser.

WASHINGTON — As then-Vice President Joe Biden left the West Wing each evening to head home, he often popped his head into national security adviser Susan Rice’s nearby suite of offices to check in — sometimes on pressing foreign policy matters, sometimes just to shoot the breeze.

“My favorite unannounced visitor was Vice President Joe Biden,” Rice wrote in her book “Tough Love.”

In those casual visits, as well as in daily national security briefings, Biden and Rice forged an easy working relationship, according to people who worked alongside them during their eight years in the Obama administration. It’s that personal relationship, and Biden’s firsthand knowledge of how Rice would operate in close proximity to a president, that are now seen as among her strongest attributes as Biden considers her to be his running mate on the Democratic presidential ticket.

Rice was initially seen as a surprise candidate for the role. The 55-year-old has a long pedigree in foreign policy, but has never held elected office. She is close to former President Barack Obama and his network of policy and political advisers, but has a lower public profile than other women Biden is considering. She has also been a lightning rod for criticism from Republicans, who contend she put politics ahead of national security.

Yet Rice has quietly gained support among some Democrats in recent weeks as Biden’s advisers grapple with the enormity of what he would inherit if he defeats President Donald Trump in November.

Rice is seen as a No. 2 who could shoulder much of the early workload on foreign policy while Biden focused on the COVID-19 pandemic and reviving the economy.

And though Rice briefly flirted with running for Senate from Maine this year, she’s seen as less likely to be eyeing the presidency herself — a bonus among some Biden advisers who worry about speculation that the 77-year-old Biden might only serve one term if elected. Some of Rice’s supporters also note she didn’t challenge Biden for the Democratic nomination in 2020, drawing a pointed contrast with Sens. Kamala Harris and Elizabeth Warren, who have also been vetted for the No. 2 spot.

But it’s the close working relationship Rice forged with Biden during the Obama administration that is seen as a key intangible that other contenders simply don’t have.

“His entire theory of politics is personal relationships,” Ben Rhodes, who served as Obama’s deputy national security adviser, said of Biden. “The idea of him taking a leap of faith on someone he really doesn’t know because it seems to check a political box seems very unlikely to me.”

Biden is in the final stages of selecting his running mate, and is believed to soon be holding in-person conversations with a short list of contenders. He’s said his running mate will be a woman, and is considering several women of color.