(CNN)With all the focus on a slew of new polls that show President Donald Trump falling further behind former Vice President Joe Biden, one number has largely slipped through the cracks -- and it is a blaring warning signal to Republicans on the ballot this fall.

       Here it is: 51% of registered voters said they would prefer a Congress controlled by Democrats in 2021, while just 40% preferred a Republican-controlled Congress, according to a new Wall Street Journal-NBC poll released Sunday. That marks a change in Democrats' favor from January when the party had just a 6-point edge over the GOP on what is known as the "generic ballot" question.

       (Side note: it's called the "generic ballot" because no specific candidates named are used. Just the two parties.)

       More from CNN Politics

       The hidden history of the secret presidential bunker VIDEO: Martial law: Here's what can and can't happen in the US Fact check: Five false or misleading claims Trump and his allies make about the response to the protests

       The generic ballot has long been used by political handicappers as a sort of blunt instrument to understand voter sentiment. Think of it as a sort of weather vane that tells you which way the political winds are blowing and, roughly, how strongly.

       Traditionally, small edges for either party (3 points or under) on the generic ballot tend to indicate small-ish gains in Congress for that side. But once one side has a double-digit lead in the generic ballot, look out! That tends to correlate with much larger seat switches.

       Read More

       So, for example, CNN's last poll prior to the 2018 election showed Democrats with a 53% to 42% edge in the generic ballot question. Democrats netted 40 seats and retook the majority days later. In the 2010 midterms, the last CNN poll showed Republicans with a 6-point lead in the generic. They won 63 seats and the House majority.

       Given that history, you can see why Democrats' current double-digit edge in the generic ballot should be of real concern to Republicans with an eye on winning back the House majority and keeping their edge in the Senate.

       "Little doubt an election held today would be a Biden landslide/GOP wipeout," tweeted Cook Political Report House editor David Wasserman on Monday. "But the amount the world has changed in the past five months should caution us how much could look different five months from now."

       THE POINT -- NOW ON YOUTUBE!

       In each episode of his weekly YouTube show, Chris Cillizza will delve a little deeper into the surreal world of politics. Click to subscribe!

       True -- and true! The 2020 election isn't for another 148 days (and yes, I am counting).

       But when you combine the generic ballot with national and swing state polling on the presidential race, the clear concern coming out of the White House in regard to the state of the Senate and the massive financial edge for House Democrats over their GOP counterparts, you begin to see the making of a Democratic landslide -- if the political environment remains roughly the same in November as it is today.

       "Even after losing 40 seats in 2018, there's no guarantee Republicans won't lose more in November," wrote Nathan Gonzales and Jacob Rubashkin in the late May edition of Inside Elections, a political handicapping site. "With less than six months to go before Election Day, not only is the House majority not at risk, Democrats could gain seats. Right now, the most likely outcome is close to the status quo and fall into a range of a GOP gain of five seats to a Democratic gain of five seats."

       A look across the political landscape as of today produces this grim reality for Republicans: It could be very ugly out there come November.

       Paid Content

       “ExpressVPN十分可靠 - 试用30天“ expressvpn

       U.K. Prime Minister Boris Johnson Sells London Home Mansion Global

       Cristiano Ronaldo Selling Former Manchester Mansion for ￡3.25M Mansion Global

       Learning a new language this year? – This app gets you speaking in just 3 weeks Babbel

       Politics Trump Homeland Security official says he believes George Floyd…

       CNN International - Videos Fox News anchor apologizes after offensive image airs

       World Another mysterious radio burst in space is repeating a pattern. This…

       Business Fed economist pushes for unprecedented rate cut in the…

       Politics Fact Check: McEnany makes false claim about Romney's share of…

       Sean Connery’s Former Home in the South of France Lists for €30 Million Mansion Global

       Ellen DeGeneres and Portia de Rossi Flipping Montecito, California, Home Mansion Global