Political journalists are fond of metaphors, and one recent analogy that seems to be rising in general usage is the comparison of the presidential nomination process to a highway with multiple "lanes" corresponding to identifiable party factions or subgroups. According to this view, each candidate and primary voter resides in a specific party lane (or, on rare occasions, can straddle the boundary between two lanes). The best-positioned candidates in the race, then, will be those who can unite the voters in their lane—either because they have it all to themselves from the start, or because they quickly knock similarly-situated candidates off the road.
It's not surprising that the "lane" concept gained popularity during the initial stages of the 2016 Republican nomination contest. With so many candidates running that they couldn't even fit on a single debate stage (seventeen in all, including at least five or six with plausible paths to the nomination at various points), some sort of classification scheme seemed necessary to make sense of the situation. One representative Washington Post analysis from early 2015 (prior to Donald Trump's entry into the race) identified four Republican lanes: Establishment (led by Jeb Bush, Scott Walker, and Marco Rubio), Social Conservative (home to Rick Santorum, Mike Huckabee, and Ben Carson), Tea Party (dominated by Ted Cruz), and Libertarian (aligned with Rand Paul).
In 2020, it's the Democrats who will have a large and varied field of candidates, and so analysts are already getting to work defining the salient subcategories within the party and figuring out where each potential contender stands in relation to them. One conceptual framework might emphasize ideology: Elizabeth Warren and Bernie Sanders on the party's left edge; Michael Bloomberg and Amy Klobuchar on the moderate wing opposite them; Kamala Harris, Cory Booker, and Kirsten Gillibrand jostling to occupy the middle space in between. Or, perhaps, the supposed lanes in the Democratic race more closely correspond to boundaries of social identity like race and gender, with voters lining up behind candidates who share their demographic characteristics. Or maybe the press will decide that the contest is really a story of Democrats who prioritize economic concerns facing off against Democrats motivated more by cultural causes, or a battle of generations, or even (please, let us be spared from this again) beer drinkers versus wine drinkers.
While some of these analytical attempts to sort out the primary competition contain grains of truth—there are, after all, identifiable constituencies within the parties that are more or less attracted to various candidates—the "lanes" model of characterizing nomination contests is fundamentally flawed and potentially misleading. It rests on assumptions about how voters behave in party primaries that don't hold up in reality, as the history of presidential nominations (including the 2016 race) makes very clear.
A reliable rule of thumb about nomination politics is that when voters are required to make an electoral choice among multiple candidates within the same party, their preferences will be relatively weak, unpredictable, based on limited information, and open to change up until the moment they cast their ballots. It can be easy to impose a clever and plausible-sounding analytical structure on the process in advance, or to explain in retrospect why one candidate won more support than another. But in the midst of the action, there is plenty about nominations that resists straightforward interpretation or forecasting. And the larger the field of contenders, the more complicated things get.
Candidates bob up and down in the polls on waves of positive or negative media attention (five different Republicans held the lead in national surveys at various points between October 2011 and February 2012, according to the RealClearPolitics aggregator). Expectations about which opponents will benefit when a particular candidate suffers a collapse in support frequently turn out to be mistaken. The important differences separating the various candidates in the eyes of party voters are themselves open to perpetual contestation by the candidates themselves, and may shift over the course of the race. And past nominees have often attracted broad support within the party by finessing internal differences in order to court multiple constituencies at once, even at the cost of logical incoherence—such as Barack Obama's self-portrayal in 2008 as simultaneously more principled and more open to compromise than his opponent Hillary Clinton.
Even though the "lanes" analogy originally caught on as a way to conceptualize the Republican nomination contest in 2016, it didn't turn out to capture the dynamics of the race that year—and may have even lulled some Republicans into adopting an ineffective or counterproductive strategy. Heading into the Iowa caucus, a widespread belief held that most Republican voters were resistant to nominating Donald Trump (and, perhaps, Ted Cruz as well), but the "establishment" lane was clogged with too many candidates: Bush, Rubio, Chris Christie, and so forth. Once a single contender broke out of the pack, Republican regulars would likely coalesce around him, and he would be in a good position to overtake Trump.
This assumption is why rival Republican candidates spent more time criticizing each other than attacking Trump despite his lead in the polls, and why Rubio's third-place finish behind Cruz and Trump in Iowa attracted a burst of media hype ("here, finally, is the establishment's chosen horse!"). But Rubio stalled in New Hampshire (thanks in part to Christie's decision, following the same strategic premise, to attack him instead of Trump in the next debate), and Trump's victory there started to set him on a path to the nomination. Rather than bumping against a hard ceiling of support, Trump's vote share in primaries and caucuses started to approach an outright majority as more Republicans jumped on the bandwagon of a successful candidate. Just as in past nomination contests, doing well in Iowa and New Hampshire generated favorable publicity for Trump that led to electoral momentum, and winning in one set of states made it easier to win in the next set as his popularity grew across the supposed boundaries separating one party subgroup from another.
It's important to understand how candidates behave strategically to build electoral coalitions and, to the best of our ability, to identify what considerations prompt voters to choose a specific candidate. But any conceptual model of nomination politics needs to incorporate a large random error term, representing the varying effects of personal charisma, persuasive advertising, memorable debate performances, catchy slogans, journalistic takedowns, verbal gaffes, and other factors that have proved difficult to anticipate yet can be just as influential as substantive positions or group membership in shaping voters' evaluations of the candidates. We're about a year away from primary and caucus participants being asked to officially register their preferences, which means that we're still a year away from rank-and-file Democrats beginning to settle on their choice of nominee. It's a long road to the nomination, and the vagaries of timing and luck ensure that many unforeseen twists and turns still lie far ahead.