Apply For U.S. Citizenship While Your I-751 Is Pending (cummingslawpractice.com)
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Suppose you have submitted Form I-751 with the USCIS, you have yet to receive approval of the application, and you are still waiting on your physical 10-year green card. In that case, you may still be eligible to pursue the Naturalization process by submitting Form N-400, Application for Naturalization, even while your I-751 application is still undergoing processing.
However, it is essential to remember that the USCIS will not approve your Form N-400 without first approving your Form I-751."