This British poll taken by an independent group found the likely reason Trump's poll numbers are declining recently

In early June, the politically independent public policy research organization, Democracy Institute,  President Trump was ahead of former Vice President Joe Biden by 48 percent to 47 percent with 5 percent undecided. It was a poll of 1500 likely voters, cell and landline, that was done by an automated system randomizing the questions. They found equal numbers of Republicans and Democrats..As of April 2020, Gallup polling found that 31% of Americans identified as Democrats, 30% identified as Republican, and 36% as Independent. Of course the numbers could have changed in the last three weeks. The polling has. RCP has Biden with a +9.4 lead now.  Most of the news media and many pundits claim Trump's support is collapsing due to the rioting and looting making Americans feel less secure and driving them to Bide. That seems contra logical to me. Biden hasn't even spoken out against the rioters that I can find. Instead, he has supported protestors while mostly hiding in his basement. 

So, what is another possible explanation? The British poll by the Democracy Institute has a strong candidate.

However, only 37 percent of Trump voters would want their friends and family to know how they had voted while 74 percent of Biden supporters are comfortable with it being known, suggesting there is a “silent vote” for the President.

President Trump's supporters have been driven underground by the relentless attacks from the press, liberals and Democrats, but I repeat myself. The Democracy institute poll didn't force those polled to talk to an actual human.This NBC/PBS/Marist Poll did and found Biden up by 8 points. The most recent (2 days ago) Hill-HarrisX poll, which is conducted online, found Biden only up by 4. The NY Times/Siena poll that found Biden up by 14 also uses live interviewers.

The data would seem to suggest the hidden Trump supporter part of the electorate has increased.

 

Hillary Clinton below 50% in latest 2016 poll...

Fading fast...

Former Secretary of State Hillary Clinton's fumbles and bumbles in the rollout of her latest memoir, Hard Choices, has knocked the wind out if her easy sail to the White House and put her support in key 2016 presidential election matchups below 50 percent for the first time.
At a time when she should be padding her lead over the top Republican contenders, her underwhelming effort has cut her numbers, said pollster John Zogby.
In his latest Zogby Analytics survey taken June 27-29, for example, her commanding leads over former Florida Gov. Jeb Bush, Kentucky Sen. Rand Paul and New Jersey Gov. Chris Christie have shrunk greatly, and she doesn't even receive 50 percent of the vote. Worse, her support among married and wealthy voters has plummeted.

Polling Shenanigans...

In the days before and during the RNC convention, major news media sources conducted eight polls. Since the close of the RNC over a week ago, only one poll by CNN/Opinion Research has been taken and released. It showed a Romney-Obama tie and a plus two bounce for Romney from a poll taken ten days earlier. Of course Rasmussen and Gallop are updating their daily tracking polls and both showed a small post convention bounce for Romney. Rasmussen found a 4 point improvement in likely voters. Gallop only found a 1 point improvement among registered voters. None of the other news outlets and polling organizations even checked to see if there was a Romney post convention bounce. It almost like they don't want to report anything positive for Romney. I call Shenanigans on this journalistic failure. Here is a bold prediction, Every major news organization in the country will be polling like crazy this weekend, when they typically find less republicans, hoping to be able to tout an Obama post convention bounce. Stay tuned.

Via RCP: