Loyal Opposition is the concept that one can be opposed to the actions of the government of the day without being opposed to the constitution of the political system.
In the United Kingdom and many other Commonwealth countries the leader of the party possessing the second-greatest number of seats in Parliament is termed "The Leader of Her Majesty's Loyal Opposition". Their constitutional function is to scrutinise Government legislation and actions. Whilst frequently opposing Her Majesty's Government at every turn, the leader of the opposition is not opposed to Her Majesty's right to the throne.
This is an important difference between the developed constitutional monarchy of the 18th century onwards and the previous squabbles between competing candidates for the throne supported by different religious and economic groups.
Most developed political systems have a similar concept though without the British ritual jargon. Parties acknowledge the constitutional system in which they operate, including the procedures to change the constitution itself. A change of government can take place without revolution and without revolutionary implications.
In the United States, the most common application of the term is to refer to the major political party (Democratic or Republican) which does not hold the office of President during time of war (most notably the Republican Party during World War II), implying an obligation for said party to cooperate fully and without reservation in the war effort. It is rarely if ever used in that country during peacetime.
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