They did. I'm not sure they still do.
Remember, the economy was a big Trump selling point. That's a bust.
At most Trump still has the white supremacists and the Evangelicals that are still hoping to get one more antiabortion SCOTUS seat.
40% of voters is not a winning number even with the EC. You have to get those middle undecideds.
You're right, 40% of the electorate isn't a winning margin unless you can get a significantly larger proportion of that 40% to vote than the other 60% and/or you can split that 60%.
Just spitballing here but let's pretend that turnout would otherwise be 100% (as opposed to the 50%-60% it really is).
One sixth of that 60% are Republicans who are absolutely sick of President Trump but they cannot bring themselves to vote Democrat - they just stay at home - now it's 40% Trump, 50% not-Trump and 10% not vote.
Another tenth of the 60% are Libertarians, Greens, Socialists and other people who won't vote for Trump, but they wont' vote for Biden either - now it's 40% Trump, 44% Democrat, 6% Third Party and 10% not vote.
Now let's say that the GOP push their voter suppression efforts into overdrive. They purge voter rolls, they make postal voting nigh-on impossible in Democrat-leaning districts (whilst making it really easy in GOP-leaning ones) and they have as few polling stations as possible in Democrat-leaning areas and/or have "unfortunate" issues with the voting infrastructure which means that people would have to queue for hours to vote - many can't afford to do this and are disenfranchised. This could affect another one tenth of the non-Trump group - now it's 40% Trump, 44% Democrat, 6% Third Party, 6% can't vote and 10% not vote.
Despite having only 40% support, the GOP gets 47% of the votes cast to the Democrats' 45%.
Heck, I'm pretty sure that the Democrats will have a significant lead over the GOP in the popular vote but still risk losing the Electoral College.