What is the test for establishing a new claim in a subsequent federal habeas petition?

Absent cause and prejudice or a fundamental miscarriage of justice, new claims raised in subsequent habeas petitions are barred as an abuse of the writ. McCleskey v. Zant, 499 U.S. 467, 493-94 (1991). In order to demonstrate cause, petitioner must show that " 'some objective factor external to the defense' " prevented petitioner from raising the claim in his prior federal petition. Id. at 493 (quoting Murray v. Carrier, 477 U.S. 478, 488 (1986)). In order to meet the fundamental miscarriage of justice exception, petitioner must supplement his constitutional claim with a colorable showing of factual innocence. Id. at 494.