Who can be sponsored to become a US citizen?

No one needs sponsorship to become a US citizen. Foremost criteria for becoming a US citizen is that you need to be a green card holder first.Any one who is above age of 18 years and has been in US as a green card holder for the past minimum 5 years can go ahead and apply for US citizenship by filing Form N-400. In case you are born in US to US parents then you automatically become a US citizen by birth and you would just need to file Form N-600 and get the citizenship certificate. Also in case you get married to a US citizen the alien spouse is granted conditional status valid for 2 years immediately which can be upgraded to regular green card by filing Form I-751 exactly 90 days before the conditional status expires provided the alien is still married to the same American.Subsequently after upgrading and getting green card and staying in US for just 3 years the spouse can apply for US citizenship.