“But it was his attacks directed at people who could not respond on an equal footing — either because they do not share his power or stature or because professional responsibility precluded them from engaging at such a level — that revealed Mr. Trump as unworthy of being our president.”

Collins’ defection from her party’s presidential nominee means she joins a group of roughly 20 percent of her fellow Senate Republicans refusing to support Trump.

Included in the group that has pledged not to support Trump are Sens. Lindsey Graham of South Carolina, Mark Kirk of Illinois, Ben Sasse of Nebraska and Dean Heller of Nevada.

A handful of other Republicans have not endorsed Trump — but have not ruled out supporting him. That group includes Sens. Ted Cruz of Texas, Jeff Flake of Arizona, Cory Garner of Colorado, Mike Lee of Utah, Lisa Murkowski of Alaska and Pat Toomey of Pennsylvania.