WASHINGTON—The GOP establishment is growing increasingly agitated that Donald Trump appears to be cruising to the nomination, yet mainstream Republicans lack a concerted, well-funded attack plan four months after he leapt to the top of the polls.

A handful of scattered, uncertain efforts to try to stop Mr. Trump—one super PAC recently compared him to a hippopotamus—have failed to dent his popularity. And a number of 2016 rivals and GOP leaders appear reluctant to wage war with Mr. Trump for fear of alienating his supporters,...