A constitutional attorney is an attorney who files lawsuits when there is a question of constitutionality. Constitutional attorneys practice in every country in which a constitution is the chartering document of the country. This means that there are constitutional lawyers in Canada and the United States, for example, but not in Britain where there is no formal written constitution.
When a constitution exists, it is the supreme law of the land. A constitution can exist on the state level or the federal level. This means, for example, that the United States Constitution governs the entire United States. Each state is under the control of the United States, but can also write its own constitution that applies within that state.
Because a constitution is the supreme law of the land, all other laws and rules must not violate the constitution; that means they cannot be unconstitutional. If a law is passed by the legislature that may violate the principles contained in the constitution, plaintiffs affected by that law can sue. A constitutional attorney would handle the case.
A constitutional attorney would also handle litigation that arises from a plaintiff's belief that his constitutional rights have been violated. For example, if a plaintiff believes he has been unfairly arrested for protesting in violation of his constitutional right for free speech, that plaintiff would hire a constitutional lawyer to sue the police force or the individual who arrested him.
Cases arising from the federal Constitution are normally brought before a federal court. If the question raises particular interest and either the plaintiff or defendant appeals, the case could go before the Supreme Court. In the United States, for example, when a plaintiff sued a school alleging that the presence of the words "under God" in the Pledge of Allegiance was a violation of his constitutional right to freedom of religion, a constitutional attorney represented that plaintiff.
Constitutional lawyers play an important role in safeguarding the freedoms within a country. Under separation of powers and rules of jurisdiction, courts cannot hear a case unless there is an actual controversy. In other words, the court can't just say that they think a law is unconstitutional; a plaintiff must sue and challenge the law and the court then can make a ruling on whether the law violates constitutional rights or not.
When the constitutional attorney represents a plaintiff and brings the case before the court, the court then has a chance to rule on the law. If the law is found unconstitutional, it is struck down as invalid. This provides an important check on the legislative branch's power.