Restatement (Second) of Contracts § 241 gives the general rules for identifying a material breach.
There are special rules for material breach in the context of a statement or action that indicates that one will fail to perform a contractual obligation.
The special rules about material breach define when a repudiating statement or action constitutes a material breach. When a repudiating statement or action constitutes a material breach, it is, under the definition in Restatement (Second) of Contracts § 250, a repudiation: a repudiation = a repudiating statement or action which constitutes a material breach.
Note: to add the complication, the Restatement uses "repudiation" for a repudiating statement or action and for a repudiation = a repudiating statement or action which constitutes a material breach. Be on guard for this when you read the Restatement.
Working through Hochster v. De La Tour will considerably clarify the special rules and how they work.