Trump and Biden even in Iowa is one of the most consequential polls, largely because it makes Joni Ernst re-election chances slimmer. Selzer Co.'s latest poll shows Greenfield up +3 on Ernst, and Selzer is considered the most accurate pollster in the US.
These polling numbers keep getting better and better for Biden. Even the notoriously Red-favored Rasmussen polls showed Trump with a -13 approval rating (43% disapprove, 56% disapprove), the largest negative gap for him in years in Rasmussen. I think people are taking this the right way too... ALmost nobody is taking this as a slam dunk, unlike 2016, where people read into slim polling advantages for Clinton and translated those "70% likelihood" as "100% Likelihood."
What's great about struggling numbers in conservative states is that it means Trump has to spend money and time advertising there. Just two or three weeks ago, they had to start advertising in Ohio, a state that TRump easily won and was expected to easily win again in 2020, but then polls suggested Ohio was a toss-up... SO hey have to start spending. And now, you look at Arkanasas, Iowa, Georgia, Texas... Down the line and they end up having to shift funding and coordination from a state like Michigan, Wisconsin, and Pennsylvania over to those conservative states just to ensure that they don't lose one of them... It makes winning Michigan, Wisconsin, and Pennsylvania that much more possible.
For the senate, the influence is stronger. These senate elections are all going to be about turnout for Republicans, and as support waines for Trump, it becomes harder for those Republican senators to hold their seats. Right now, Arizona and Colorado seem solid blue pickup. MAine is increasingly becoming more likely. Montana is leaning blue. And Iowa has just become +3 for D from a reliable pollster. THat's your senate majority right there even if you assume Democrats lose the hard-to-keep Alabama Doug Jones seat (But even that's a wildcard... Like if Sessions threatens to stay in the race against Tuberville, all Doug Jones needs is for ~5% of Tuberville's base to write in Sessions or not vote and it's a competitive race). Those 5 seats give Democrats a senate majority, and those are all looking increasingly likely. And then, if there's an unexpected upset in any of the "stretch goal" seats -- Ossoff in Georgia, McGrath in Kentucky, the person challenging Lindsey Graham, North Carolina, etc. Then you're not just looking at a 51/49 majority, but an actual solid senate majority if you can steal a seat. If Trump turnout is low and Democratic turnout is high, then that becomes more and more likely.
Trump teeters at this 40% support precipice, dropping below 40% in some polls which he hasn't had happen since the first months of his presidency, in the wake of Charlottesville, firing Comey, Mueller appointment, and so on. John McCain famously said that the magic number for Trump to lose Republican support publicly is 35%, if he drops below 35% his support will crumble from the Republican establishment and more will come out against him. In the next 2 months we have John Bolton's book dropping (like, in 7 days) and a critical book by Trump's niece in August. He also has these Supreme Court case announcements coming soon, within the next few weeks, in which 2 look like they'll be a personal loss for him. If his support falls below 35% I really think that the house of cards topples.