Injury or prospective injury confers standing - people with no legal right to be present sue every day in courts. If legislation granting something very valuable to them would pass but for an unconstitutional obstruction, that’s perfect standing. The case should be dismissed on other grounds, that it is a political question, which is lawyer talk for courts abstaining from interpreting how the Senate exercises its constitutional power to make and enforce its own rules.
This case should be thrown out for lack of standing on the part of the illegal aliens and separation of powers ground for the Congresscritters.
What grants them the right to that standing? The constitution grants citizens the right, but WHAT exactly grants illegals the right to sue ANYONE in US court?
What you just said sounds very nice, but it seems like an assumption to me.