WASHINGTON (AP) — Worried about "Republican-on-Republican violence," top party donors are taking action, with one firing off a letter calling for more civility and another seeking to block businessman Donald Trump from the debate stage altogether. Foster Friess, a Wyoming-based investor and one of the party's top 20 donors in the last presidential contest, issued a letter to 16 White House prospects and the Republican National Committee late last week calling for candidates to stay on the "civility reservation." The call for calm comes as the sprawling Republican field shows signs it could tip into a bare-knuckles struggle for the nomination — a scenario that the party's elite donors see as a distressing echo of four years ago. While few party officials see the reality television star as a credible candidate, he has lashed out at a growing number of Republican critics who have condemned his recent description of Mexican immigrants as criminals and rapists. Trump stood firm on his comments about immigrants, saying earlier Monday "the Mexican government is forcing their most unwanted people into the United States," and "criminals, drug dealers, rapists" are among them. "Ninety-nine percent of leading donors saw the candidates carve each other up in the 2012 primaries and come out weaker for it and are determined not to let that happen again," said Fred Malek, who has helped raise money for GOP presidential candidates for four decades.