Is there anything in the U.S. Constitution that would prevent two states, with clear consent from their electorate (approval by both state legislatures, statewide referenda), and with said consent explicitly forfeiting two of their Senators and two of their Electoral Votes, from merging into one? Say, maybe, North and South Dakota decide that for whatever reason, they’d prefer to just be one big state of “Dakota”? Does the Constitution grant the Federal government or any neighboring states any say in the matter?
A follow-up question, equally hypothetical: would there be any difference if the two states that wished to merge were Maryland and Virginia? Assuming the merger agreement would stipulate that Washington, DC is not a part of the combined state, is there anything in Federal law or the founding documents of the District of Columbia that says the district must not be contained within the borders of a single state?