The short answer is no.
An "enemy" in the context of the phrase "enemy combatant" is a term of art that refers to a group defined by Congress with whom the United States is at war or against whom it is authorized to use military force. In recent U.S. history, the Authorization for Use of Military Force (AUMF) (Public Law 107-40) passed by Congress following 9-11 has defined who qualifies as an "enemy" as it is the only "live" declaration of war in force at this time.
For example, the Declaration that supported the Padilla detention as an enemy combatant was as follows:
In accordance with the Constitution and consistent with the laws of
the United States, including the Authorization for Use of Military
Force Joint Resolution (Public Law 107-40);
I, GEORGE W. BUSH, as President of the United States and Commander in
Chief of the U.S. armed forces, hereby DETERMINE for the United States
of America that:
(1) Jose Padilla, who is under the control of the Department of
Justice and who is a U.S. citizen, is, and at the time he entered the
United States in May 2002 was, an enemy combatant;
(2) Mr. Padilla is closely associated with al Qaeda, an international
terrorist organization with which the United States is at war;
(3) Mr. Padilla engaged in conduct that constituted hostile and
warlike acts, including conduct in preparation for acts of
international terrorism that had the aim to cause injury to or adverse
effects on the United States;
(4) Mr. Padilla possesses intelligence, including intelligence about
personnel and activities of al Qaeda, that, if communicated to the
U.S., would aid U.S. efforts to prevent attacks by al Qaeda on the
United States or its armed forces, other governmental personnel, or
(5) Mr. Padilla represents a continuing, present and grave danger to
the national security of the United States, and detention of Mr.
Padilla is necessary to prevent him from aiding al Qaeda in its
efforts to attack the United States or its armed forces, other
governmental personnel, or citizens;
(6) it is in the interest of the United States that the Secretary of
Defense detain Mr. Padilla as an enemy combatant; and
(7) it is REDACTED consistent with U.S. law and the laws of war for
the Secretary of Defense to detain Mr. Padilla as enemy combatant.
Accordingly, you are directed to receive Mr. Padilla from the
Department of Justice and to detain him as an enemy combatant.
DATE: June 9, 2002 Signature
In the context of prior cases about "enemy combatants", the precedents used the declarations of war from World War II, and in the Civil War, respectively to determine who was an "enemy". The two pre-9-11 cases that were used to support the enemy combatant doctrine were Ex Parte
Quirin, 317 U.S. 1 (1942) (World War II), and Ex Parte Milligan, 71
U.S. (4 Wall.) 2 (1866) (U.S. Civil War).
A U.S. official would have to establish in a Declaration under penalty of perjury that an individual was associated with one of the terrorist organizations covered by the AUMF to declare someone an enemy combatant. The AUMF does not apply to "terrorist organizations" in general.
For example, a self-proclaimed terrorist from the Animal Liberation Front could not be detained as an enemy combatant, but that is not among the terrorist organizations that Congress has declared war on in the AUMF.
It is also established from a variety of post-9-11 cases that anyone so detained would have, at a minimum, a right to seek their freedom via a petition for habeas corpus from which they or someone on their behalf, could petition to a U.S. District Court to seek their release.
The issue is not citizenship, it is affiliation with an enemy designated in the AUMF.
A U.S. citizen whom a U.S. official could declare under penalty of perjury was affiliated with the AUMF probably could be detained in this fashion, although neither the Padilla case nor the sister Al-Marri and Hamdan cases involving non-U.S. citizens, ever finally reached the U.S. Supreme Court on the merits of the question of whether the enemy combatant doctrine can be applied within a U.S. State (the U.S. Supreme Court case of Rumsfield v. Padilla, 542 U.S. 426 limited itself to the question of the proper venue for a habeas corpus suit), which was one of the key issues before the courts in those cases.
It is not at all clear how the courts would resolve that issue outside the 4th Circuit. The Padilla case in 2005 did give rise to a precedent in the United States Court of Appeals for the 4th Circuit that supports the practice, although a subsequent ruling in the Al-Marri case arguably muddied the waters.
The subsequent developments in the enemy combatant cases (some of which arose collaterally in the criminal trial of Padilla), haven't really change the relevant law. There have also been military commission cases since Padilla, but they go to the question of the proper scope and process to apply in military commissions of enemy combatants imposing some punishment beyond indefinite detention as a quasi-prisoner of war for the duration of the war on terrorism authorized by the AUMF.