While diversity of citizenship may be an appropriate base for jurisdiction,  42 USC 1983 is not, at least in Texas  There, the attorney is not a state actor:

"In Combs v. City of Dallas, 3:06-CV-0074-P, 2006 U.S. Dist. Lexis 92445 (N.D. Tex. 2006), the client sought to sue the attorneys who represented him during his state and federal criminal prosecutions. The court held that neither appointed nor retained counsel acts under color of state law in representing a defendant during criminal proceeding. See Polk County v. Dodson, 454 U.S. 312, 324 (1981) (public defender does not act under color of state law when performing a lawyer’s traditional functions as counsel to a defendant in a criminal case); Mills v. Criminal Dist. Court No. 3, 837 F.2d 677, 678 (5th Cir. 1988) (court appointed counsel are not official state actors); Russell v. Millsap, 781 F.2d 381, 383 (5th Cir. 1985) (retained counsel does not act under color of state law). The same rationale applies to appointed or retained counsel in a federal criminal case. McLeod v. Knowles, 2006 WL 1738286, *1 (5th Cir. 2006) (unpublished per curiam) (extends Polk County v. Dodson to a Bivens action against court-appointed counsel). As such the conduct of criminal defense attorneys in representing a federal criminal defendant is not cognizable under 42 U.S.C. § 1983 or as a Bivens action.

The client alleged that one of the attorneys had conspired with the prosecutors. Assuming that this sufficiently alleged action under color of law, the court nonetheless found that the legal malpractice claim was barred because it inherently challenged the validity of the client’s conviction:

In Heck v. Humphrey, 512 U.S. 477, 486-87 (1994), the Supreme Court held that a party may not maintain a civil rights action based on the legality of a prior criminal proceeding unless a state court or federal habeas court has determined that the terms of confinement are in fact invalid. This rule applies equally to Bivens actions. Stephenson v. Reno, 28 F.3d 26, 27 (5th Cir. 1994). The critical inquiry is whether a judgment in favor of the plaintiff in the civil action would "necessarily imply the invalidity of his conviction or sentence." Heck, 512 U.S. at 486-87. If so, the claim is barred unless the conviction has been reversed or declared invalid. Id.

[. . . Plaintiff’s criminal conviction has not been reversed on direct appeal, expunged by executive order, or called into question by a federal writ of habeas corpus. . . . ] "