Both are veteran members of that exclusive club called the United States Senate - Biden heading the Foreign Relations Committee and McCain forging a campaign finance reform effort. Both are old hands on the presidential campaign trail, looking for a better result this fall than they had in past election cycles.
Those ties made it less than surprising that McCain, the Republican candidate for president, picked up the phone Saturday morning to call his old pal, just named the running mate of Democratic candidate Barack Obama.
"It was a brief call between friends," said McCain spokesman Tucker Bounds. "John McCain simply called to congratulate him and let his friend know he was thinking about him and his wife."
That's how things work in the Senate, where shifting alliances are the order of the day and today's ally can be tomorrow's rival.
There's considerable history between the two, and a lot of shared years in Washington.
Biden was elected to the Senate from Delaware in 1972 at the age of 29, taking office the following year after reaching the requisite age of 30. He's a six-term veteran who sought the Democratic presidential nomination in 1988 and again this year, fading fast both times.
McCain is a four-term senator from Arizona who moved over from the House in 1986, and he sought the Republican presidential nomination in 2000.
They've forged alliances over the years, and compliments have often flowed between them. McCain angered many in his own party by forging a bipartisan agreement to avoid filibusters over judicial appointments, drawing praise from some Democrats. Biden stood behind President Bush - along with McCain - when Bush signed the War Powers Act in 2002.
Biden suggested in the 2004 election cycle that McCain could build ties with Democrats. McCain's maverick status may have alienated some Republicans, but it made him appealing to Democrats, Biden noted.
"I think that this is a time for unity in this country and maybe it is time to have a guy like John McCain - a Republican - on the ticket with a guy he does like," Biden said, referring to 2004 nominee John Kerry. Kerry and McCain share a Vietnam War history.
A deep interest in foreign affairs has shaped both McCain and Biden, and they've found times to agree. In 1999, the two co-sponsored a measure authorizing the use of "all necessary force and other means" in Yugoslavia.
With the war in Iraq defining the current foreign policy debate, both have sounded warnings about President Bush's policies in very similar ways.
"One of the very big mistakes early on was that he didn't have enough troops on the ground, particularly after the initial victory and that's still the case," McCain said.
"There's not enough force on the ground to mount a real counterinsurgency," Biden said after visiting Iraq and rejecting Bush's arguments.
Yet a shared time in the Senate and a willingness to work together hardly means the two are unwilling to chart partisan courses.
After losing a bitter primary battle with Bush in 2000, McCain became Bush's eager surrogate in the 2004 election.
Biden offered Kerry his views on the war in Iraq and how to deal with the issue on the campaign trail. Those same views will now go to Obama.