Winnow: verb. To expose (grain or other substances) to the wind or to a current of air so that the lighter particles (as chaff or other refuse matter) are separated or blown away.

— Oxford English Dictionary

WASHINGTON — It is time to dust off this marvelously appropriate verb for its quadrennial use to describe the thinning of a field of presidential aspirants. After two rounds of quasi-debates — "10-participant debate" is a quasi-oxymoron — the Democratic field is well on its way to contraction.

Joe Biden survived his second debate, but did not dispel the impression that the brittleness of his candidacy is more important than his double-digit lead in a field the congestion of which is, for now, his friend. He has never been the Democrats' Demosthenes. Now, however, when he commits the sort of verbal fender-benders that have long characterized him, or when he has a normal hesitancy reaching for the mot juste or an elusive fact, many people will wonder whether he is showing his age, 76.

There is not room for both Elizabeth Warren and Bernie Sanders in the lane reserved for those who think, eccentrically, the government is a tool of rapacious factions and that the government should be made much more powerful. If Warren can be more of a happy warrior, and less of the faculty-club know-it-all scold who showed up in Detroit, she should send Sanders packing.

Regarding John Delaney, Steve Bullock, John Hickenlooper and some others who are still a far cry from double digits in polls, remember this: In January 1972, South Dakota Sen. George McGovern's support was around 3%, which means he was within the margin of error of zero. Six months later he clinched the nomination.

Many Democrats, who understand that their policies will remain mere aspirations if Republicans retain control of the Senate, are exasperated that three of their aspiring presidents are not seeking Senate seats next year. Hickenlooper, a former two-term Colorado governor, could be trying to deny a second term to Sen. Cory Gardner, the most vulnerable Republican incumbent. Bullock, having won Montana's governorship twice in a state Trump carried by 20.6 points, would be a strong opponent against Sen. Steve Daines.

Some Democrats wish that former Texas Rep. Beto O'Rourke would run against three-term incumbent Sen. John Cornyn, but O'Rourke, the incredibly shrinking candidate, always has been a figment of his and others' imaginations.

This might be 2020's decisive paradox: The safer that continuing Republican control of the Senate seems on Election Day, the better are the Democrats' chances of winning the White House. Many voters, perhaps a decisive number, will be willing to put a progressive in the presidency if, but only if, they know that they can count on that which they too often deplore: gridlock.

George Will's email address is georgewill@washpost.com.