On Friday, Hillary Clinton’s choice of Tim Kaine as her running mate projected confidence. By Monday, it looked more like overconfidence.

For more than a month, Mrs. Clinton and her allies have been running campaign ads in battleground states, and Mr. Kaine, a senator from one such state (Virginia), is a potential plus in nearly all of them. His bilingualism could enhance her appeal among Hispanic voters in places like Florida and Colorado. His Roman Catholicism could help her in swing states with large Catholic populations such as Wisconsin and Pennsylvania. As a former governor of Virginia, Mr. Kaine brings executive experience and regional appeal in adjacent North Carolina, a state that Barack Obama carried in 2008 and that could be in play again this cycle.

Tapping Mr. Kaine also demonstrated that Mrs. Clinton was looking past Election Day. The senator has been a member of the Foreign Relations Committee and is respected on both sides of the aisle, which could come in handy if Democrats win the White House but not control of the House and Senate. Mrs. Clinton wanted someone with a centrist reputation who could increase her appeal among independent voters and disaffected Republicans who can’t stomach Donald Trump.

But for the Kaine choice to be met with minimum blowback, at least two preconditions had to be met. First, supporters of Sen. Bernie Sanders, the Vermont socialist who dogged Mrs. Clinton all the way to this week’s Philadelphia convention, had to be sufficiently appeased. Second, Mrs. Clinton had to convince throngs of liberal activists obsessed with racial and ethnic diversity that her choice of a white male was not a snub. This is where Team Clinton may have become too confident.

Right up until Mr. Kaine was chosen last week, Democratic strategists were stressing the party’s “unprecedented” unity. Mr. Sanders’s endorsement of Mrs. Clinton and changes made to the party’s official platform had more than satisfied progressives like Sen. Elizabeth Warren of Massachusetts and Black Lives Matter activists, they insisted. Moreover, President and Michelle Obama will campaign vigorously for the Democratic ticket, and their popularity among black voters means Mrs. Clinton doesn’t have to worry about black turnout.