A former school board candidate failed to convince a Missouri appeals court that Husch Blackwell LLP filed a bogus defamation suit to “silence” his criticism of a municipal bond campaign that the firm’s client was underwriting.

The court’s April 23 opinion highlights the difficulty of proving two types of tort claim—malicious prosecution and abuse of process—that give disgruntled litigants a shot at turning the tables on opposing parties and their lawyers.

The ruling is also likely to end a contentious...