For Americans with dual citizenship, holding a policy level office in another country's national/federal government is (at least theoretically) deemed to constitute constructive renunciation of U.S. citizenship.
Holding a policy level office in another country's government is considered an "expatriating act". If one has committed an expatriating act, one can make a claim with the U.S. Dept. of State (DoS) that U.S. citizenship was voluntarily relinquished effective the date of the commission of the expatriating act (DoS
may recognize that claim). On the other hand, DoS
may itself assert that citizenship was lost effective the date of the commission of said act, despite the wishes of the citizen. It's also entirely possible that a citizen may remain a citizen, despite having committed such an act -- however, one's U.S. citizenship
may be revoked at any future date, at the pleasure of DoS, retroactive to the date of the commission of said act.
These days, however, being an American is like being the member of a mob family, they tend to never let you out (a bit hyperbolic, perhaps, but for those who do, so to say, get out, in a way that amounts to being dead to them).
Once FATCA and FBAR regulations began to be enforced more stringently and foreign governments/institutions began adhering to these regulations, more and more Americans living abroad found themselves caught up in this reporting dragnet, sometimes with life altering consequences. This resulted in a flood of "relinquishment" claims by citizens who claimed to have voluntarily relinquished their citizenships by having committed potentially expatrating acts (e.g., naturalizing as a foreign national) with an intent to relinquish citizenship. DoS took the position that many of these claims were frivolous or fraudulent, and tried to strong-arm citizens to relinquish citizenship solely via "renouncement" procedures. DoS then significantly raised the fees for "renouncement" (probably to pay for the sudden increase of D-I-Y relinquishment claims). Currently, as a matter of course, DoS seems to be rejecting most relinquishments that were not processed via renouncement procedures (e.g., filing an official form, paying a fee, swearing an oath before an authorized agent, and being issued a certificate with a date certain for loss of U.S. citizenship).
It should be noted that current DoS policy on relinquishments (both renouncements and DIY relinquishments) is largely in response to FATCA and FBAR reporting. It is entirely possible that DoS may one day return to a policy of deeming self-reported relinquishments as valid. It may even more actively revoke citizenship from those who have committed potentially expatriating acts if they can find evidence to support an intent to relinquish in such acts (e.g., Facebook/blog posts).
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Relinquishment: a citizen may relinquish their U.S. citizenship by committing (i) an expatriating act, (ii) a potentially expatriating act with an intent to relinquish citizenship; or (iii) acts so inconsistent with an intent to maintain U.S. citizenship, that an intent to relinquish is presumed.
There are two ways to relinquish citizenship: (a) relinquishing by committing any of the above acts and being prepared to present evidence of said acts, and one's intent at the commission of such acts, to DoS; or (b) renouncing by filling out the proper forms, paying the filing fees, presenting oneself and swearing/affirming before a government agent authorized to accept such oaths/affirmations, and receiving a certificate that includes a date certain of loss of citizenship. Both are valid ways to relinquish citizenship, but only a renouncement guarantees DoS recognition and provides a date certain of loss of citizenship. (American citizens who would be left stateless after relinquishment must "renounce" their citizenship.)