Jeb Bush was supposed to be doing better than this. He had name recognition, funding, experience and the respect of the Republican establishment. But his numbers have been in the basement for months, while Donald Trump has steadily remained the front-runner for the GOP presidential nomination.

Despite the large gap between their polling numbers, the Bush-Trump dynamic has become one of the most fiery relationships among the remaining Republican contenders (the brief pre-Iowa Trump-Cruz feud aside).

It’s easy to see why Bush would attack Trump, but Trump’s open and continued public disdain for Bush is a little tougher to understand. The latest RealClearPolitics poll average has Trump beating Bush by more than 25 percentage points. Why would the runaway leader in the national polls spend so much time attacking a candidate polling in the single digits?

In his first rally since the New Hampshire primary on Tuesday, Trump attacked Bush at least eight times, while barely acknowledging the other remaining GOP candidates (Trump did also attack Hillary Clinton and Bernie Sanders). He used his signature “low-energy” attack on Bush, and of course went after the former Florida governor over his position on Common Core Standards.

Trump might see Bush as a threat, despite the polls. Bush has always been Trump’s best-funded Republican opponent, and was supposed to be the “establishment” front-runner (remember that?). And Bush does go after Trump more directly than most other GOP candidates, calling the New York businessman a bigot, a fraud and a false conservative. He even went as far as saying that the United States would be worse-off under Trump than it is under President Obama — just about the worst insult one Republican can hurl at another.

With more debates on the horizon, and the GOP field as muddled as ever, the Trump-Bush feud isn't likely to end anytime soon. It might not be over until one of the two drops out of the race — and the way things are going now, that could still be months away.