It has been obvious for days that opponents in Congress would be unable to derail the agreement between Iran and six world powers to rein in the Islamic Republic’s nuclear program in exchange for relief from debilitating economic sanctions. When Sen. Barbara Mikulski, D-Md., became the 34th senator to endorse the agreement, President Obama had all the votes he needed to sustain a veto if Congress passed a resolution disapproving the deal.

But on Tuesday the number of senators backing the deal passed 40, meaning that Obama may not have to veto anything. Instead, his Senate allies could use a filibuster to stop the resolution from ever coming to a vote. That may not happen; some senators who support the agreement may nevertheless decide that the Senate should vote on it directly. Yet the mere possibility of a filibuster has provoked outrage from Republicans. Senate Majority Leader Mitch McConnell said senators are “entitled to an up-or-down vote — not a filibuster or artificial limits on passage.”

McConnell’s objection is unpersuasive. So is a suggestion by Sen. Tom Cotton, R-Ark., that the Senate’s top Democrat, Harry Reid of Nevada, “wants to deny the American people a voice entirely by blocking an up-or-down vote on this terrible deal.”

Whatever you think of the agreement with Iran — this newspaper has endorsed it despite its imperfections — it’s hard to see why it should be treated any differently than any other piece of legislation, vital or trivial.

Under the current rules of the Senate, that means that 60 votes are needed to end a filibuster and move to a final vote on a contested bill. (As an alternative to such a vote, Reid proposed a deal to require 60 votes to pass the resolution of disapproval.)

This page has long criticized the use of the filibuster to block both presidential appointments — a practice mostly ended in 2013 — and legislation. But so long as 60 votes are required to cut off debate, we see no reason why this matter should be an exception.

If the Republicans who control the Senate believe that the 60-vote requirement stifles the voice of the American people, they should move to alter the rules across the board.

Republicans worry that the public will lack confidence in an agreement that is preserved by the “back door” of a filibuster. Some would say the same if the deal were saved by a presidential veto that Congress failed to override. But neither of those outcomes was foreclosed by the legislation that gave Congress a role in this process while preserving the dominant role of the president in foreign affairs.

In the end, the legitimacy of this agreement will depend on whether it succeeds in its objectives, not on the parliamentary process by which the United States agreed to it.