Here's why, courtesy of a chart from RealClearPolitics detailing the polling averages for Clinton and Trump over the past three months.
Trump's numbers shot up in the wake of his victory in the May 3 Indiana primary, a win that effectively sealed his nomination. But as May wore on, Trump's poll numbers not only hit a wall, they began to collapse.
Why? It's always difficult to pinpoint a single reason. But it is worth noting that on May 27 Trump gave his 11-minute speech bashing U.S. District Judge Gonzalo Curiel. He has now spent the better part of the past month trying to defend, then explain, then defend not only those comments but also why questioning Curiel's Mexican heritage was all just a big misunderstanding.
What's fascinating in the chart above is that Clinton's appeal hasn't surged as Trump's has dipped badly. These aren't people deciding to be for Clinton; it's people deciding they aren't for Trump. The more Trump is exposed to a general electorate — at least in the early days of his time as the presumptive Republican nominee — the less that electorate likes him. (Seven in 10 voters in a new Washington Post-ABC News poll said they have a negative opinion of Trump.)
There is, of course, plenty of time between now and Nov. 8 for Trump to turn things around. But there is very little evidence he (a) sees any problem or (b) feels it at all necessary to overhaul his approach. Trump has repeatedly insisted that he won't change anything about himself in a general election. And with each passing day, he shows that his brash and bullying persona — and his willingness to clash with fellow Republicans — isn't a glitch, it's a feature.
The problem for Republican elected officials who worry that Trump's tanking numbers might — if they continue to plunge — cost the party much more than the White House is that they have no other options. Trump won the Republican primaries fair and square. He got 13 million votes. He competed in all 50 states. There's no debate that he is the one candidate (out of the 17 who ran) that Republican base voters want as their nominee.
If his polling slide continues, Trump could well be an anchor that Republicans in office across the country simply can't shed — even though they know it will drag them to the bottom.