The biggest electoral blowout in US history was Lyndon Johnson's win in 1964. The popular vote was 61-38.5. Polls that force people to choose rather than say undecided show about a 14 point race now with Biden up near 60%. That indicates that the undecideds are people who have left the Trump camp, but haven't decided whether they are going to vote for Biden or not.
I've been watching the approval polls for almost 4 years now. When you look at the breakdown of approval, Trump's strong approval has always hovered in the 20-25% range while his strong disapproval hovered in the 50% range. The strong approve and strong disapprove are people who have solidly made up their minds about his re-election. The mild approve and disapprove are "gettable" under the right circumstances. trump is currently bleeding the 20-25% mild approve people who have now decided they don't approve of him anymore. Some have jumped on the Biden bandwagon while others are trying to decided whether to vote Biden, third party, or stay home. They have decided Trump doesn't deserve their vote again.
Considering how close presidential elections usually are, this one is beginning to look like one of the bigger blowouts.
Ultimately elected Republicans want to keep their jobs. Very few of them really like Trump. I have heard from many sources that if they thought they could get re-elected by getting rid of Trump, almost every Republican Senator would be climbing over one another to finish him off. If Trump's support collapses, the elected Republicans will turn on him.
He desperately wants COVID-19 to go away. It's been an unmitigated disaster for Trump.
You point out the real weak spot in US elections: the inside man (or woman) exploit. In person voter fraud or trying to mail in multiple ballots is retail hacking, it picks up one or two extra votes and while some people have done it to prove a point, trying to do it on any scale almost always leads to getting caught, as shown in the arrests today in Los Angeles. The more people involved in a conspiracy, the harder it is to cover up. The chances of one person mentioning something incriminating to the wrong person goes up every day the conspiracy exists. That's why criminal gangs have such draconian penalties for people who betray their secrets. They need to to maintain security.
One person might be able to manage to cast a couple of extra votes, but they are risking getting caught of a fairly major crime in most states for only a minuscule gain.
The place to commit voter fraud on a scale that makes a difference is to have someone on the inside at the office that counts the votes or to rig the voting machines in places that have electronic voting. That's the best bang for the buck and a very small number of people can swing elections with lower risk of getting caught.
All this noise being made about in person voter fraud and mail in voter fraud is a distraction to divert attention from the real vector for voter fraud.
And anyone who understands how the GOP works has pointed out that Trump's mission to destroy the reputation of mail in voting is just going to hurt the GOP because the bulk of permanent mail in voting for the last 30 years has been reliable GOP voters. The GOP GOTV efforts has involved targeting likely GOP voters and getting them signed up to permanently vote by mail.
In polls asking people about whether they would prefer to vote in person vs vote by mail (before the pandemic), people who tend to vote Democrat preferred in person voting because they got something out of the ritual. GOP voters just wanted to get it done and vote by mail was easy.
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Republican Secretary of State had an interview on NPR a few months back about this very topic. There have been several studies done on the frequency of in person voter fraud was in both polling places and by mail. The instances of it are tiny because it's a very high risk for low reward crime.
Lessons To Learn From Washington's Decades-Long Experience Of Mail-In Voting
Why steal pennies when you can get your guy inside and steal millions? In places like Washington they also have security measures to protect the ballots once they are in the system. The signatures on the ballots are digitally scanned with state of the art signature recognition software and anything rejected is spit out so humans can inspect. There are various checks to prevent one human or a few humans from tampering with the results.
But some states are not great at security in their election offices. I've read Florida is one of the worst at it.