What is the standard of proof required to prove that a professional attorney was negligent in connection with litigation?

between the breach and the resulting injury; and (4) actual loss or damage resulting from the attorney's negligence.' " (Ambriz v. Kelegian (2007) 146 Cal.App.4th 1519, 1531 (Ambriz).) " '[F]ailure to prove . . . any of [the elements] is fatal to recovery.' " (Namikas v. Miller (2014) 225 Cal.App.4th 1574, 1581 (Namikas).) The trial court in the underlying proceeding found the causation element in Nassi's legal malpractice claim "conclusively negated"; as such, the question presented here is whether the evidence established the absence of any triable issue as to causation.

In a legal malpractice cause of action, "the method for proving the element of causation has been likened to a 'trial within a trial' or a 'case within a case.' " (Ambriz, supra, 146 Cal.App.4th at p.1531.) This trial-within-a-trial or case-within-a-case approach " ' "is the most effective safeguard yet devised against speculative and conjectural claims . . . . It is a standard of proof designed to limit damages to those actually caused by a professional's malfeasance." ' " (Namikas, supra, 225 Cal.App.4th at p. 1582.) " '[A] client claiming that his [or her] attorney was negligent in connection with litigation has . . . the difficult task of demonstrating that, but for the negligence complained of, the client would have been successful in the prosecution or defense of the action in question.' " (Sukoff v. Lemkin (1988) 202 Cal.App.3d 740, 744.)