A good running mate has to help their ticket accomplish two things: be effective members of the administration, and ensure that they get to the White House in the first place. But the latest V.P. chatter swirling around Hillary Clinton is leading some Democrats to worry she’s focused too much on the former and not enough on the latter. For months, Virginia senator Tim Kaine has been discussed as a top pick for the Democratic ticket—speculation that has reached a fever pitch in recent weeks. Now, with the loudmouthed, unpredictable Donald Trump already slinging mud at Clinton over her husband’s sexual history, some Democratic operatives are signaling their concern that Kaine might be too mild-mannered to play the attack-dog role.

In any other year, Kaine would have been a solid pick. Kaine, the former governor of Virginia and chairman of the Democratic National Committee, has been on several V.P. short lists since 2008, and was likely at the forefront of Clinton’s mind back in 2014 when he supported her pre-candidacy. Of course, this was all prior to the Republican Party falling in line behind Trump, a bully who requires an equal and opposite force to be neutralized. That could be Elizabeth Warren, who has seemed to relish in publicly berating Trump in recent weeks. But it’s not as clear if Kaine is up to the task. As a former colleague from the D.N.C. put it to Politico, “What he’s not is an attack dog. . . . It’s not his natural disposition. He’s a thoughtful guy who likes to play in the world of facts and policy. He doesn’t shoot from the hip, and he’s not a sound-bite master.”

There are some signs that Kaine could learn to play Trump’s game: the senator recently made waves for a floor speech in which he accused the Senate Republicans of obstructing Barack Obama’s Supreme Court pick, Merrick Garland, because of the president’s race. “In what way is this president so different from all who came before to justify such treatment?” he asked, pointing out that Obama was elected twice and yet the G.O.P. refused to even consider his nominee. It was a classic Kaine move, allies say. “He is really able to fillet you before you realize it, and before you know it, you’re on the grill,” his media advisor, David Eichenbaum, told Politico. Kaine is also the rare blue unicorn who can connect with multiple Democratic interest groups. He is a religious, white southerner who can speak Spanish fluently and has attended a predominantly black church for more than two decades, all while sharing a similar agenda to Clinton (but with a greater fluency in social-justice issues). But if Kaine’s talents are to be of any use to Hillary Clinton, he would need to go against an opponent who can actually feel shame.

Nor does this election cycle, which has been defined by anti-elite rage on both sides of the aisle, play to Kaine’s strengths. With Bernie Sanders’s campaign railing against the D.N.C., and many of their supporters disinclined to unify behind Clinton, tapping a former D.N.C. chair as a vice-presidential candidate would inevitably reinforce Clinton’s image as an establishment politician and consummate party insider. In any other year, Kaine would have ticked off all the boxes that a good V.P. would require, but in 2016, the Democratic base is looking for more than a white career politician who previously served as the party’s chief fundraiser. More important, in an election that demands pithy sound bites and social-media-friendly theatrics, the subtle Kaine perhaps lacks the same passion that have made Warren and Sanders so popular. This really is not Kaine’s year.