What is the burden of proof for a "settle and sue" legal malpractice action?

The parties have not cited, and we have not located, any constitutional provision expressly stating the burden of proof for legal malpractice actions or, more specifically, for the elements of causation and damages in a "settle and sue" legal malpractice action. In addition, the parties have referred to no authority establishing or suggesting a higher level of proof than the preponderance of the evidence is implied by the state or federal due process clause. We mention the due process clauses because they are usually the constitutional source of a heightened burden of proof. (E.g. Santosky v. Kramer (1982) 455 U.S. 745, 747748, 102 S.Ct. 1388, 71 L.Ed.2d 599 [in proceeding to terminate parental rights, Due Process Clause of the Fourteenth Amendment requires state to support its allegations with clear and convincing evidence].)