If you are applying for U.S. citizenship through the process known as naturalization, using Form N-400, there are a number of grounds upon which you can be denied. (We're assuming that you're not using a more obscure route to U.S. citizenship, such as requesting automatic citizenship through your parents' U.S. citizen status using one of the legal processes called acquisition or derivation.)
The grounds for denial of a Form N-400 might be based on:
An application for U.S. citizenship gives the U.S. immigration authorities another chance to review your entire file. Occasionally, USCIS encounters instances where the applicant really shouldn't have been given a green card.
For example, let's say an applicant immigrated as the unmarried child of a U.S. citizen, but was in fact married when he or she immigrated. This applicant might, on the N-400 application, unthinkingly put down his or her actual marriage date. If USCIS notices that this date was before the applicant's receipt of the green card, it may ask further questions, and ultimately place the applicant in removal proceedings.
Once stripped of the green card, the applicant will have no more right to U.S. citizenship.
Green card holders are subject to the grounds of deportability found in Section 237 of the Immigration and Nationality Act (I.N.A.), or 8 U.S.C. Section 1227.
For instance, you can become deportable for having committed certain crimes or immigration violations, engaged in subversive or terrorist activities, or even failed to advise USCIS of your change of address within ten days of moving.
If you happen to have spent six months or more outside the United States and then returned, you were also subject to the grounds of inadmissibility upon your return, which cover not only various crimes and violations, but the issue of whether you have become a public charge (received need-based government assistance) or developed certain physical or mental disorders that present a threat to the public or yourself.
If, during your application for citizenship, USCIS notices that you have become removable or were inadmissible upon entry to the U.S., it can place you into removal proceedings and you might lose your green card (lawful permanent residence).
Citizenship is the highest benefit a person can obtain under U.S. immigration law, and therefore comes with a list of requirements. These include that you:
Failure to meet any of these criteria can mean that your application for citizenship will be denied. In most cases, you will not lose your green card at the same time, but will simply need to apply for citizenship when you are ready. (However, there are exceptions, for example if you were out of the U.S. for so long that you appear to have abandoned your residence here.)
To avoid being denied on basic eligibility grounds, fill out your N-400 application carefully, and consult with an attorney if you have any crimes on your record, have spent long periods of time outside the United States, or have any other questions about your eligibility for U.S. citizenship.