No, I think the real parallels are with other mid terms such as 2006 and 2010.
First, let's just dispense with something here: Trump's rally attendance isn't a good sign of his support. For one thing, it's not out of proportion to what other losing candidates have had. Second, rally attendance is only really indicative of how many people you can turn out at a rally. That's different than how many people will vote for you, and even the largest rally is just a tiny fraction of how many people will vote in an election. Third, you've got to remember that Trump lost the popular vote in 2016. He didn't win because he was popular and got lots of people to love him. He won despite being the least popular presidential candidate ever and because of a fluke of the rules.
Now, on to the analogies. I think the real analogy is to other mid term elections, specifically the 2006 and 2010 elections. In both of those elections, the people who backed the president were in complete denial about the mounting evidence that the opposition was going to have a good year. In 2006, the Bush supporters had convinced themselves that, despite Bush's problems in his second term, including a worsening Iraq War and a disastrus handling of Katrina, that they would hold on to power forever. Bush's political strategist, Karl Rove, predicted that the Republican party not only wouldn't lose control of Congress but would in fact gain seats in the election. When reporters pointed out that the polls seemed to point to the opposite outcome he said "You've got your math, but I've got THE math!" That was the day before the election. He proved to be disastrously mistaken. Republicans took "a thumpin" in Bush's words, and lost both the House and the Senate in one of the biggest mid trm landslides in history.
Four years later it was Democrats turn to be overconfident. Two years earlier, Obama had won a major presidential victory and Democrats had crushed it in the Congress, gaining even more seats in the House and coming within one vote of a filibuster proof majority in the Senate. They were absolutely sure that they'd do well in the mid terms and that people would reward them for saving the economy and enacting a massive expansion of health care. They dismissed all the evidence of a red wave as faulty polls that didn't reflect what Americans really felt. They were also horribly wrong. Republicans didn't win the Senate, but they won back the House in what was, IIRC, the largest single election gain of House seats in history.
Republicans seem to be in the same boat now. They can't seem to conceive that the same Americans who elected Trump two years ago would actually elect Democrats to Congress this year. History proves you guys mistaken. That could absolutely happen. The big difference now is that while Bush and Obama actually got the majority of votes, Trump did not. That makes your insistence that Republicans will win this year seem especially foolhardy.