Canadian citizens cannot acquire U.S. citizenship without first having a green card for a certain period of time. As outlined in Getting a Green Card.
When a Canadian citizen obtained permanent residency through marriage to a U.S. citizen, he/she can apply for naturalization three (3) years after becoming a permanent resident.
When a Canadian citizen obtained permanent residency through employment or though a family member, he/she can apply for naturalization five (5) years after becoming a permanent resident.
If you were born in Canada but you have a U.S. citizen parent, you may also qualify for U.S. citizenship through naturalization.
Continuous residence requirement
It is not enough that you are a permanent resident for three or five years before seeking U.S. citizenship. You must also satisfy a continuous residence requirement, which means that you have to physically reside within the U.S. for a certain period of time in the years prior to applying for citizenship.
For green cards through marriage, the Canadian citizen must have been physically present in the U.S. for at least 18 months (1.5 years) of the 3 years prior to applying for citizenship while on a green card.
For green cards through employment or family, the Canadian citizen must have been physically present in the U.S. for at least 30 months (2.5 years) of the 5 years prior to applying for citizenship while on a green card.
Generally, you should not leave the U.S. for a continuous period of more than six months. Doing so may break your continuous residence time unless you can demonstrate that you continue to work, reside, pay taxes and have ties to the U.S. If you leave the U.S. for a continuous period of one year or more, you may be deemed to have abandoned your permanent residency and lose your accumulated continuous residence time. If you plan to leave the U.S. for a year, but to return afterwards, it is recommended that you apply for a re-entry permit prior to your departure. This will show the government that you intend to return and that you do not intend to abandon your permanent residency status.
In addition to the physical presence requirement, Canadian citizens must also reside in the state or district from which they are applying for at least three months before seeking U.S. citizenship. Applicants must also demonstrate good moral character, a good understanding of the English language, and a loyalty to support and to defend the U.S. Constitution.