Every 10 years
Any Article III (constitutional) court (or equivalent state court) can declare a law unconstitutional if the law is part of a case they're trying or reviewing; however, the government would probably appeal the case all the way to the Supreme Court to get a definitive answer.
The US Supreme Court is the ultimate arbiter of constitutionality, and has final authority over questions of constitutionality.
Article III Courts
The Line Item Veto Act was signed into law by Pres. Bill Clinton on the 9th of April 1996. The line-item veto was designed to thwart the attempts of those who would attach unrelated items to bills, especially "pork barrel" items, by giving the Pres. the power to pass some parts of a bill and veto other parts. However, the U.S. Supreme Court declared the act to be unconstitutional, pointing out that the Constitution limited the President's choices to accepting or rejecting a bill.
Government officials can't accept positions without Congress' consent.
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