President Donald Trump has tumbled through the floor of his approval rating, according to CNN's Harry Enten, and history provides some painful lessons for him and fellow Republicans.The latest polling finds the 79-year-old president well below 40 percent, which Enten told "CNN News Central" was a perilous position for the Republican Party looking ahead to November's midterm elections."There's been all this talk about Donald Trump has this floor," Enten said. "Well, I'll tell you, that floor looks like it was built by the worst carpenters I've ever seen, because he just keeps falling through that floor, and it's not just the New York Times [poll]. This is what I think is so important. Yes, the Times poll got people talking, but it just follows a slew of polling data that all shows the same thing, and that is that Donald Trump is not just under 40 percent. He's well under 40 percent in terms of his approval rating, 37 percent. New York Times, CBS came out on Sunday, 37 percent, Ipsos, 36 percent, our CNN poll, 35 percent.""You put it all together and it is just very clear at this point that Donald Trump is hitting new lows in his polling data," he added, "and this one poll is no outlier – it is part of a much larger trend."Enten found only two other examples of presidents who fell below 40-percent approval at this point in their second term, and he said they provide cautionary tales."One was Richard Nixon back in 1974, [and] his party absolutely got crushed in the 1974 midterm elections, suffered huge losses, House and Senate," Enten said. "How about George W. Bush back in 2006, his party lost the House, his party lost the Senate again, 30-seat loss in the House for George W. Bush in 2006, a six-seat loss in the United States Senate.""So historically speaking, of course, we'll have to just wait and see what happens here," he added. "But historically speaking, when you have a president who has a sub-40 approval rating at this point, their party goes down to defeat in the midterm elections."History also shows that presidents who fall as low as Trump has don't usually recover, Enten said."The answer is no," he said. "Nixon, of course, was out by August of '74. He ended up with, what, a 24 percent approval rating, and George W. Bush. You know what happened to him by the end of his term in 2009, he was actually in the high 20s. He wasn't just sub-40, he was in the 20s. Both Bush and Nixon ended up in the 20s in approval rating.""Again, I'm not saying that's going to happen with Donald Trump," Enten added. "This is a sample size of two, but this is history that you definitely don't want to be a part of. You do not want to be part of this brotherhood." - YouTube youtu.be