A well-funded group of anti-Trump conservatives has sent its donors a remarkably candid memo that reveals how resilient former President Donald Trump has been against millions of dollars of negative ads the group deployed against him in two early-voting states. The political action committee, called Win It Back, has close ties to the influential fiscally conservative group Club for Growth. It has already spent more than $4 million trying to lower Trump’s support among Republican voters in Iowa and nearly $2 million more trying to damage him in South Carolina. But in the memo — dated Thursday and obtained by...