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Trump is behind in a state no Republican has won the presidency without in 96 years

(CNN) Poll of the week: A new CNN/SSRS poll finds that former Vice President Joe Biden leads in the state of Florida by a 51% to 46% margin over President Donald Trump among registered voters.

The CNN poll follows a Quinnipiac University poll released earlier this week, which showed Biden with a 51% to 38% lead.

What's the point: Trump hasn't led in a single Florida poll since early March.

The fact that the polls in Florida favor Biden should be one of the biggest warning signs yet for Trump's fledgling campaign. Yes, we still have 100 days to go, and history does suggest that the gap in Florida could close.

Still, Florida is probably the bellwether state that most meets the definition of "must win" for Trump if he wants to be elected to a second term, and he is losing there.

No Republican has won the presidency without Florida since Calvin Coolidge in 1924.

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Moreover, it's a state that leans a little bit to the right of the nation. The last time the state voted more Democratic than the nation as a whole in a presidential election was 1976. The fact that Trump is down here by an average of 8 points in high quality live interview polls since June 1 suggests he is down significantly nationally.

Biden, on the other hand, has a clear path to 270 electoral votes without Florida. Biden has held 6 to 12 point leads in polls released this week from Michigan and Pennsylvania. This includes 6 point and 12 point advantages in Michigan from CBS News/YouGov and CNN/SSRS polls respectively released on Sunday. High quality polls from June gave Biden an average 10-point lead in Wisconsin. If Biden adds all of those states to his column plus the 232 electoral votes from the states Hillary Clinton won in 2016, he gets to 278 electoral votes.

Winning Florida gives Biden a lot of backup options given that it's worth 29 electoral votes. If Biden adds the 29 electoral votes from Florida to the states Clinton took in 2016, he gets 261 electoral votes. Biden would need just 9 electoral votes more to get an electoral college majority. He could add any other state that Trump won in 2016 by 9.0 points or less.

As I noted a few months ago, Florida is geographically and demographically diverse from the Great Lake battleground states. If Biden stumbles in the mostly White Great Lake swing states, he could conceivably hold onto Florida and add on the diverse swing state of Arizona. Biden has consistently been ahead in Arizona, and he was up 4 and 5 points in the latest CNN/SSRS and NBC News/Marist College polls out Sunday.

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Additionally, Biden could just win one of those Great Lake battleground states and Florida to get to 270 electoral votes. Biden could, for example, add Michigan (16 electoral votes) to his column, and it would be enough. Biden has held the advantage in every single nonpartisan poll in Michigan since early March.

Perhaps as importantly for Democrats, the polling in Florida has generally been accurate at the end of the campaign. There hasn't been an error like there was in the Great Lakes in 2016. The final Florida polls from CNN have been within 3 points of the outcome in every presidential election since 2008. The same holds true for the gubernatorial and Senate elections in 2018.

With Biden's polling lead being as wide as it is right now in the Sunshine State, the past accuracy of the final polls suggest he really is ahead right now.

The good news for Trump is that history does indicate how difficult it would be for Biden to win the state by a large margin. The last time a Democrat won the state by more than 6 points was 1948. No candidate from either party has won the state by more than 6 points since 1992.

(That's an even longer streak for close elections than the infamous bellwether of Ohio. Unlike Florida, Ohio really isn't a bellwether state anymore as indicated by Trump's 1-point advantage in a CBS News/YouGov poll out on Sunday. Biden was up 10 points in a national CBS News/YouGov poll also released Sunday.)

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Overall the point is that we shouldn't be surprised if the margin in Florida closes down the stretch. That's exactly what happened in the 2018 midterms, when Republican candidates for governor and Senate squeaked out wins by less than a point.

But for now, Florida is emblematic of larger challenges Trump faces. It's been a state ravaged by the coronavirus, which has almost certainly contributed to Trump's problems in the state.

As I've said many times before, Trump likely can't win if he doesn't turn around his low approval ratings on the coronavirus. His approval rating in Florida on the issue is just 42% among registered voters in the latest CNN poll.

Were that to remain the case through Election Day, Biden's likely the next president.