The new initiative is the brainchild of Ms. Longwell; Bill Kristol, the conservative writer; and Tim Miller, a former top aide to former Gov. Jeb Bush of Florida. Together, Ms. Longwell and Mr. Kristol have also worked on an initiative called Republicans for the Rule of Law, which has begun its own ad blitz against Mr. Trump.

After almost three years of conducting focus groups and intensive research on messages that would work with persuadable voters, the founders have created a cache of 100 testimonial videos, most shot on smartphones, with voters explaining why they are making the sometimes painful choice to break with their political party.

Some of the videos are hardly rousing endorsements for Mr. Biden. In one testimonial, Wayne from Dallas says to the camera, “I could not bring myself to vote for Hillary, so I voted for President Trump.” But he said he believed the president had “gotten worse” and that “everything he’s done has been to enrich himself.” With a note of resignation, he says: “I will not be voting for him here in 2020. I suppose I’ll be voting for Biden.”