Someone at a port of entry who demonstrates his/her US citizenship to the satisfaction of the immigration officers has met the immigration inspection requirements and must be allowed to enter the US, even if he/she does not have a US passport or meet other entry requirements.

Under U.S. law, a U.S. citizen cannot be permanently denied entry to the United States, as the right to enter one's own country is a fundamental right of citizenship. However, arriving at a border without a passport (or a WHTI-compliant document like an EDL or Global Entry card) will result in significant delays and secondary inspection. Customs and Border Protection (CBP) officers must verify your identity and citizenship before letting you through. If you arrive by land without a passport, you may be detained for several hours while officers search databases and interview you to confirm your status. If you are abroad and lose your passport, you must visit a U.S. Embassy to obtain an "emergency passport" because airlines are prohibited from boarding you on an international flight to the U.S. without a valid travel document. So, while the border agents won't "refuse" you forever, the process of getting back in without one is an administrative nightmare.