A constitutional amendment must be approved by a three-fifths majority by both the Congress of Deputies and the Senate. If there is disagreement between the Chambers, a mixed committee will present an agreed text to both chambers for a vote. Even if this procedure fails, and as long as the amendment passed with a simple majority in the Senate, Congress may pass the amendment with two-thirds majority. One-tenth of either deputies or senators may also request, with a 15-day deadline since passage, that the amendment be put to a referendum.