The pick isn't without risk. Like Clinton, many progressives believe Kaine is too close to Wall Street. While Kaine supports the Dodd-Frank legislation that imposes major regulations on the financial industry, he also was one of 70 senators to recently sign a letter to the Consumer Financial Protection Bureau asking for looser regulations on regional banks and credit unions.
A devout Catholic, he's also said that he's personally opposed to abortion, which has alarmed some pro-choice advocates even though he has a long record of supporting abortion rights.
Kaine describes himself as "boring" - a quality Clinton says she "loves about him" - and doesn't bring the excitement that would have come from choosing a second woman, like Massachusetts Senator Elizabeth Warren, or a Hispanic figure like Labour Secretary Tom Perez.
Clinton's calculus appears to be that he can help deliver Virginia, a vital battleground state, and reinforce her primary line of attack against GOP nominee Donald Trump: that he is too dangerous to be commander-in-chief.
On Friday, for instance, Kaine said he was "stunned" by Trump's recent suggestion that he wouldn't necessarily defend Nato allies from Russia as required under the decades-old military pact. Kaine said the GOP nominee's remarks were "very, very dangerous".
"When you say to an ally - who you have a treaty obligation to defend - 'Eh, we're not sure we will,' that is a very, very dangerous thing," the senator told reporters. "We have American men and women spread throughout those countries right now in service who are there and are at risk."
In an interview this northern spring, Kaine accused Trump of regularly insulting the US military, and said the businessman and reality TV star was "someone who wants to be commander-in-chief [and] who says the American military is a disaster".
Picking Kaine may do little to placate the progressive Democrats who flocked to Bernie Sanders, some of whom have pledged to protest Clinton during the Democratic National Convention in Philadelphia this week.
Beyond Kaine's stances on abortion and financial regulation, he also supports the Trans-Pacific Partnership, which emerged as a major wedge issue in both the Democratic and Republican primaries.
Sanders forced the former Secretary of State into opposing the international trade deal she once supported during the campaign, arguing - as Trump trumpeted over and over at the Republican convention this week - that it will cost American jobs.
These are two problems that are connected, and you can't have a strategy that's just about one
At the same time, he is no dove. Kaine argues, as Clinton now does, that the US should have intervened more aggressively when the Syrian civil war erupted more than five years ago.
Like Clinton, he has broken with the White House and supports the creation of a no-fly zone over rebel-held parts of Syria. In a December Senate Armed Services Committee hearing, Kaine said "the absence of the humanitarian zone [in Syria] will go down as one of the big mistakes that we've made," comparing it to President Bill Clinton's hesitance to intervene in Rwanda in the 1990s.
Kaine has also argued that the White House lacks a plan for ousting Syrian strongman Bashar al-Assad and has instead focused too much attention on the fight against Isis.
"There's a desire to defeat [Isis] . . . but there hasn't been a clear strategy vis-à-vis Assad," he told NPR in October 2015. "These are two problems that are connected, and you can't have a strategy that's just about one."
More broadly, Kaine has criticised what unnamed administration aides have described as one of Obama's core foreign-policy beliefs.
" 'Don't do stupid stuff.' That's not a big enough doctrine," Kaine said. "You are also often not doing stuff that's stupid not to do."
Kaine wasn't the only national security figure Clinton was considering, a sign of the importance she places on the issue substantively and as a line of attack against Trump.
- Foreign Policy
