Pope Leo XIII in his Encyclical Immortale Dei wrote that one of the “chief duties” of rulers was to “favour religion, to protect it, to shield it under the credit and sanction of the laws, and neither to organize nor enact any measure that may compromise its safety. This is the bounden duty of rulers to the people over whom they rule.” 
Ideally the State and the Church must be united. Blessed Pope Pius IX’s Syllabus of Errors condemns the proposition that the “Church ought to be separated from the State, and the State from the Church.”  Msgr. John A Ryan  wrote:
The State should officially recognize the Catholic religion as the
religion of the commonwealth: accordingly it should invite the
blessing and the ceremonial participation of the Church for certain
important functions, such as the opening of legislative sessions,
the erection of public buildings, etc., and delegate its officials to
attend certain of the more important festival celebrations of the
Church; it should recognize and sanction the laws of the Church;
and it should protect the rights of the Church, and the religious as
well as the other rights of the Church’s members. 
The above rule is not absolute. It applies to a state where the overwhelming majority of the population is Catholic.  Where Catholics are in the minority the state does not have to give official recognition to the Catholic faith and in any event an attempt to do so may be met with civil unrest.  Certainly such a condition exists in the present day United States. However, if conditions change then the “people would do wrong by voting against governmental recognition of the true religion…” 
 Leo XIII, Encyclical Immortale Dei (1885) 6.
 Syllabus, number 55.
 Msgr. Ryan was a scholar whose books were generally accepted as the orthodox view on Church-State relations.
 M. Davies, The Second Vatican Council and Religious Liberty 30 (1992).
 Macksey, C. (1912). State and Church. In The Catholic Encyclopedia. New York: Robert Appleton Company. Retrieved September 12, 2010 from New Advent: http://www.newadvent.org/cathen/14250c.htm.
 Connel, F. “Reply to Father Murray.” The American ecclesiastical review 126 (1952) : 57.