Last month, Alabama conducted a novel political experiment, conscripting the state’s Republican voters as its lab rats. The question that was officially on the ballot was whom the party should nominate for the United States Senate seat vacated by Attorney General Jeff Sessions in February. One option was Luther Strange, a former state attorney general, who had filled the seat since being appointed by a Republican governor who happened to be under investigation by Strange’s own office. The other was Roy Moore, a twice-defrocked State Supreme Court justice described last month by the conservative National Review as “nothing more than a bigoted, theocratic and ignorant buffoon,” who has argued variously that Muslims shouldn’t be allowed in Congress, that homosexuality should be illegal, that multiple Midwestern communities are currently governed by Shariah law and that the Sept. 11 attacks might have been divine punishment for America’s sins.

Political caricatures don’t come much broader than these. Strange was the establishment incarnate; Moore was the Republican electorate’s id made ruddy flesh, an avatar of the latent nativism and conspiracism that Donald Trump’s detractors inside and outside the Republican Party blamed for his rise. The only wrinkle was that the president endorsed not Moore but Strange — and the race, as Katie Glueck of McClatchy newspapers wrote shortly before the election, became “the biggest disagreement to date between Trump and the most committed elements of his base.” The outcome would be an indicator of whether Trump had really captured the Republican electorate or whether the forces that carried him to office were beyond his, or anyone’s, control.

In the end, Moore trounced Strange by nine points. National Review offered hopefully that at least the election contradicted claims that Trump had turned the party into a cult of personality; the contributor Fred Bauer wrote that the result “could be a sign that the Republican base will not be willing to roll over for the president on any and every issue.” Other observers were less delicate. “The Republican base is so nuts,” the Washington Post editorial writer Stephen Stromberg concluded, that even when Trump advised otherwise, “they had to pick one of the most divisive figures in American politics to represent them.” Jamelle Bouie of Slate argued that Moore’s win was evidence of how, “since 2008, many of those voters who comprise the Republican base have grown increasingly untethered from mainstream politics.” This was a “Jurassic Park” vision of the Republican base, in which party leaders, after fecklessly creating and nurturing a monster, find themselves powerless to stop it once the electric fences go out on the island.

A political party’s base, for much of the 20th century, usually came with an indefinite article attached: a base, rather than the base. This was a straightforward reflection of how parties operated, as sometimes lumpy and uneasy coalitions of disparate interests. In 1975, when Ronald Reagan was plotting his second run for president, it was a bipartisan assumption that the Republican Party’s existing bases — an ad hoc assemblage of WASPs, rural Midwesterners and a growing but still insufficient segment of middle-class suburbia — were too narrow and unstable for the G.O.P. to escape minority-party status.