Hey John,
Firstly, sentiment. Leo is right. Americans want a safe, calm, boring president, they are sick of the Trump's constant bullshit. Covid is probably the major issue. Trump has shown that he is unable or unwilling to deal with it and its raging across the country at the moment, especially in the North Eastern states like Wisconsin.
Secondly, fundamentals. The economy is tanked, and Trump's popularity rating is 43% and has never risen above the mid-40s.
Third, demographics. Trump has lost a significant amount of support amongst suburban women, seniors, white non college educated voters, and white voters overall in return for some very modest gains with Hispanic and Black voters. Biden is higher amongst all of the most reliable voting demographics, higher in independents and has probably taken a reasonable amount of moderate Republicans.
Fourth, early turnout. It's impossible to make definitive judgements on this but generally this unprecedented amount of turnout would be bad for an incumbent and good for the democrats and it looks like we are looking at 150-165 million total, which is nearly in record territory Texas has already reached 100% of its 2016 turnout and the youth vote seems to be up. Jon Ralston has more or less already called Nevada for Biden based on county level analysis of turnout.
Lastly, polling. As it stands Biden is at about 10+ in the national poll average. He is ahead in nearly all swing states by between +1 to +6. Most crucially, district polling, which correctly predicted Trump's victory is all pointing the right way for Biden. For Trump to win we would need to see a 2x or 3x increase in the polling error from 2016.
Trump does have a very narrow path to victory which is predicated on him maintaining all of the states he won in 2016 and then using the supreme court to cancel ballot counting in Pennsylvania and some other states that have slow counts, but the most likely result at the moment is a Biden victory, with a Biden landslide more likely than a Trump win.