A new poll finding a very tight contest in the Virginia gubernatorial race should light a fire under the backsides of Washington Democrats and prompt them to make a deal on President Biden’s agenda already. If they don’t, and a loss in Virginia follows, it could get even harder to pass.

Which underscores the deep fragility of this moment for Democrats. If the wind blows in the wrong direction in the next few days, failure could compound failure, resulting in longer-term disaster.

The new Monmouth University poll helps explain this fragility. It finds Democrat Terry McAuliffe tied with Republican Glenn Youngkin at 46 percent apiece among registered voters, a sizable shift from last month, which put McAuliffe up five points.

The poll shows each candidate ahead in different turnout scenarios among likely voters: McAuliffe leads under a high turnout model with a slightly more diverse electorate, and Youngkin leads under a slightly lower turnout model with a whiter electorate.

To be clear, we shouldn’t place too much stock in one poll. The average of polls shows McAuliffe up three points, and most polls have put him ahead.

But nonetheless, Democrats involved in the race say the Monmouth poll does capture an underlying truism about this contest. That’s because it finds Biden’s approval rating has slipped in the state to 43 percent, and that Democratic voter engagement has slipped.

This dovetails with what Democrats believe is happening, and importantly, they see these things as related: Biden’s approval is slipping in part because Washington Democrats are fighting with each other rather than delivering on his agenda, and this in turn is helping cause Democratic enthusiasm to slip.

Mark Bergman, who was senior adviser to Democratic Gov. Ralph Northam’s 2017 victory, tells me bluntly that McAuliffe’s fate could turn on whether Democrats strike a deal on the Build Back Better reconciliation bill in coming days. That’s partly because this will help decide turnout levels.

In 2017, Northam romped to a surprising nine point win, with turnout driven up to 48 percent of eligible voters, Bergman notes. That was driven in part by President Donald Trump’s supercharging of turnout in northern Virginia.

Bergman says there is no chance of replicating that turnout this time. But he believes getting an agreement on Biden’s agenda would go a long way toward boosting turnout up to at least 45 percent, which he sees as the critical threshold to enable a McAuliffe win.

There’s an interesting nuance here: Those northern Virginia precincts will be more likely to turn out in the event of an agreement precisely because they are D.C. suburbs, where the professional governing class is concentrated.

“A lot of these folks are watching Washington as if it’s their local government,” Bergman told me. “We’re going to win or lose based on how many Democrats show up in the northern Virginia media market.”

“Having the president get a win,” Bergman continued, “could be the difference between winning this race and losing.”

Monmouth polling director Patrick Murray tells me the race will likely turn on turnout in the northern Virginia suburbs relative to that in exurbs and western rural areas: If it’s high in both the former and the latter, McAuliffe is ahead, but if it’s relatively higher in just the latter, Youngkin leads.

What happens if no deal is reached, and McAuliffe loses? It will get even harder for swing district Democrats in Congress to support an ambitious deal.

“If we’re not successful in Virginia, it’s going to set off a lot of panic alarms across the party,” Bergman told me. “People are going to be looking at the president as being an anchor.”

It shouldn’t be this way — in the end, the party will rise and fall together with the success or failure of the Biden presidency. But it is a truth universally acknowledged in Washington that politicians in a party are increasingly reluctant to act on that understanding, the lower the approval of that party’s president drops.

All of which is to say that those politicians who don’t want to find themselves making such a horrible set of calculations have a real incentive to make a deal and pass Biden’s agenda already.