Forty-One Percent.

Forty-one percent.

I’ve been covering American politics for a long time and I can’t remember a number that so dramatically altered the political community’s perception of a presidential campaign as that number did, last night, at 7:30 p.m. Eastern Time.

The source of the number was The Iowa Poll, which has been the gold standard for statewide polling in the United States for decades. The number itself was the percentage of likely voters in Iowa supporting Joe Biden’s candidacy for president.

President Trump’s number was 48%, which put him ahead in the “horse race” by 7 percentage points. There was nothing really remarkable about that, in context. Mr. Trump won the state in 2016 by (roughly) nine percentage points.

What was remarkable was Biden’s 41%. What made it doubly disconcerting was the way The Des Moines Register (accurately) described the poll results:

“Republican President Donald Trump has taken over the lead in Iowa as Democratic former Vice President Joe Biden has faded…”

Faded! Could there be a more terrible word in the last week of a presidential campaign? Off the record, Democratic elected officials and campaign operatives and financial backers have been saying throughout the campaign that their biggest fear regarding the eventual outcome was Biden himself. They saw him as an especially weak candidate and worried that he wasn’t “a closer;” even if he was ahead going into the last week, victory could slip from his grasp.

Up until last night, Democratic elected officials and political operatives saw the presidential race standing at somewhere between a narrow Biden win and a “blue wave.” In their “blue wave” scenario, the Democrats would win both the presidency and a Senate majority and the Trump-McConnell nightmare would finally come to an end.

That was the other piece of bad news in last night’s Iowa Poll release. It showed that Republican Sen. Joni Ernst had pulled ahead of her Democratic challenger, Theresa Greenfield. Her lead (46%-to-42%) was within the margin of error, but it wasn’t Ms. Ernst’s lead that Democrats were focused on. It was the “faded” support for Ms. Greenfield, which almost exactly tracked the “faded” support for Joe Biden.

For Democrats, last night’s Iowa Poll was the worst possible news at the worst possible moment. It foretold close results in Wisconsin and Minnesota. It undermined the Biden campaign’s momentum and morale. And it fracked Democrats’ self- confidence.

What had seemed reasonably certain no longer seemed certain at all.