When former Vice President and presumptive Democratic presidential nominee Joe Biden tapped Sen. Kamala Harris to be his running mate, did he also effectively name his heir apparent?

The pick itself hardly came as a surprise. Harris had been the obvious choice for so long that the only mystery is why the process dragged on for as long as it did.

But it's still worth pausing to appreciate the fact that Harris — the first Black woman and the first South Asian woman to grace a national ticket — was considered the "safe" choice before considering why such an apparently historic pick is safe. She's safe because the party has changed to the point that the kind of history she's making isn't particularly astonishing. Harris is a squarely mainstream Democratic politician, not associated with any particular interest group or ideological faction, and demographically representative of the most crucial Democratic constituencies. She's charismatic, telegenic, in the prime of life, and has more substantive political experience than any of the last three presidents did at the time of their election. And while she ran an appallingly bad campaign for president, she got further than Biden himself did on his first outing in 1988.

Harris isn't going to move the polls much if at all, because nobody was going to move the polls much — unless Biden picked someone truly surprising, which could have threatened his stable lead. She is safe precisely because she doesn't threaten that lead, and is acceptable to anyone for whom the Democratic Party itself is acceptable, because she looks, sounds, acts, and votes like the Democratic Party as it actually is. Even those who want to change the party one way or another have little reason to grumble; if they can move the party (and the country) under her feet, she's the kind of politician who'll dance nimbly until she's found new solid footing.

All of those are good reasons for Biden to have picked Harris. But they are also why it's premature to start talking about Harris as Biden's heir apparent and prohibitive front-runner for 2024, as some commentators are already suggesting.

Vice Presidents are not always obvious heirs. I find it highly doubtful that Mike Pence — chosen because he represented a crucial constituency, white evangelicals — would easily rebuff the likes of Marco Rubio, Josh Hawley, Tom Cotton, and Nikki Haley if Trump had declined to seek re-election. Nor did President Reagan do much of anything to assist Vice President George H. W. Bush in his quest to succeed him. On the Democratic side, Humbert Humphrey in 1968, Walter Mondale in 1984, and Biden himself this year, all faced serious and committed opposition, and could easily have lost.

But the Democratic Party of late has gotten into a bad habit of trying to prevent a serious succession battle. In 2000, the party was overwhelmingly united behind Vice President Al Gore; his only opponent, Sen. Bill Bradley, barely got any traction. In 2008, the party attempted to unify behind Hillary Clinton, before Barack Obama made that impossible. They largely cleared the field for Clinton in her second attempt, in 2016, leaving as her only serious opponent a Vermont socialist who nobody expected to amount to much. I don't think it's an accident that these efforts, aimed at avoiding conflict and preserving unity for the general election, have largely backfired, leaving them with nominees who are unprepared to fight their way to victory. Not every contested primary produces a general election winner, but all of their successful recent nominees — Jimmy Carter in 1976, Bill Clinton in 1992, Obama in 2008 — won their nominations in a fight.

Moreover, in 2024 (or 2028), the Democrats are going to need to have a fight. For most of the 2020 campaign, the bulk of the Democratic contenders vied for support from the activist left, only to have the voters choose the candidate of calm, centrist continuity. The challenges the country faces have multiplied severalfold since then, and both the opportunities and risks of governance are much higher than they were when Biden clinched the nomination. Whether we're talking about rebuilding the economy or rethinking criminal justice or facing the twin challenges of an increasingly belligerent China and the planetary threat of climate change, the docket is astoundingly full. How can anyone blithely assume that the Biden administration will handle all of it so well that "don't change horses in mid-stream" will unquestionably be the winning slogan for his successor?

Joe Biden has called himself a bridge to a new generation of Democratic leaders — suggesting that he's offering a safe path across, and also that he knows he's not the destination. But a bridge needs an anchor at both ends. Harris still needs to demonstrate that she can be that anchor — that she has a clear and distinct vision for a country that sometimes feels on the verge of cracking up. She may demonstrate that over the next four years — but if she doesn't, she needs to be challenged.