For a century after its reorganization and renaming in the 1830s, the Democratic party had a special rule regarding selection of presidential nominees. To win the nomination at the national convention, a candidate had to receive the votes of two-thirds of the delegates.
This rule meant different things to different people. The party was originally a vehicle for Andrew Jackson, who was wildly popular among party members. But some Democrats feared that Jackson would impose his will on the party after he left office. The two-thirds rule gave a veto to minority groups within the party. The two-thirds rule was thought to mitigate sectionalism. No candidate could be nominated without some support from every section. The two-thirds rule prevented capture of the party by ideological zealots. In the first decades of the party's existence, the worrisome zealots were abolitionists and fire-breathing apologists for slavery.
The two-thirds rule worked pretty well. Its one conspicuous failure came in 1860, when it prevented the party from nominating anyone in the regular convention, and caused the splintering of the party into three parts. It was abandoned only in the 1930s, at the behest of Franklin Roosevelt, who had become the modern equivalent of Jackson, with the popularity to write whatever rules he wanted.
The important thing to note about the rule was that it was entirely a party matter. It had no standing in state or federal law. It had no effect in general elections. It was untouched by the Fifteenth and Nineteenth amendments, which transformed the electorate. It survived the Civil War and one world war. It could be restored today if the Democrats so chose.
They won't restore it and they shouldn't. Instead they should create a new rule: that Democratic nominees for president must not have reached their 70th birthday. Or 75th birthday. Or 65th birthday. Or any birthday that would ensure that the nominee was substantially younger than the current presumptive nominees of the Democratic and Republican parties.
They could do this simply as an intra-party measure, just like the two-thirds rule. It's too late to do it for this year, But it should be done to prevent a repeat of the train wreck this year's election is likely to be for the Democrats. If recent polls are even close to correct, and Joe Biden hangs on, he will be badly defeated and likely will take down the rest of the party with him. Democrats don't want that to happen. If Biden steps aside, the candidate the Democrats choose in his place will be badly burdened by the short runway Biden will have left him or her.
The heart of the problem is that the decision has been left in the hands of Biden. If a maximum age rule had been in place in the party, there would have been no decision. The rule would have been the rule. It's probably fair to say that most Democrats now wish such a mechanism were already in place. Their collective fate wouldn't be hanging on the self-interested judgment of an increasingly frail old man.
The rule should be phased in, in the way the Social Security Administration phases in increases in retirement age. The precise maximum age could be decided by a commission of the party, one similar to the commission that changed convention rules after the 1968 debacle in Chicago.
The rule wouldn't be set in stone. What the party can do, the party can undo. But the rule would be important in proclaiming the sense of the party that turnover at the upper end of age should be a regular and predictable thing.
If Biden does step aside, and especially if his successor candidate wins, the party should call it the Biden rule. If he doesn't, or if he does too late to prevent a wipeout, the Dems might have to call it the Ginsburg rule.