The free-trade deal known as the Trans-Pacific Partnership is one of the largest trade agreements ever attempted.

President Obama, who has made it one of his top priorities, says the trade deal ushers in unprecedented economic growth for countries involved.

But those against it fear it could mean more jobs will move from the U.S. to those developing countries involved.

They also do not like the fact the five-year long talks have been largely secret.

Much of the deal is being discussed in secret, which proponents say enable negotiators to press for the best deal possible for the 12 countries involved. The pact is aimed at deepening economic ties between these nations, slashing tariffs and fostering trade to boost growth. Member countries are also hoping to foster a closer relationship on economic policies and regulation. An agreement could also create a new single market like that in Europe

The countries involved are the U.S., Japan, Malaysia, Vietnam, Singapore, Brunei, Australia, New Zealand, Canada, Mexico, Chile and Peru.

The initiative began with an agreement between just four nations – Brunei, Chile, New Zealand and Singapore some 10 years ago. That deal removed tariffs on most goods traded between the countries, promised to cut more and cooperate on wider issues such as employment practices, intellectual property and competition policies.

The TPP would be much larger, however.

The 12 countries have a collective population of about 800 million – almost double that of the European Union’s single market. The 12-nation would-be bloc is already responsible for 40% of world trade.

But critics are skeptical, arguing that it could give corporations an avenue to sue governments if they don’t like the way they are being regulated.