Here's why everyone is arguing about free trade

As U.S. presidential hopefuls continue to battle through the primary season, America's trade policies have risen to the top of the political discourse.

Although there has always been opposition to free trade measures, recent decades have seen political leadership in the U.S. tending to push agendas focused on removing trade barriers between the U.S. and other countries. Both sides of the issue argue that jobs and the economy are at stake, but they disagree on the question of results and how domestic economic success is really defined.

To start with, free trade is the practice of removing restrictions on imports and exports between countries. Such restrictions can include bans, quotas and taxes among other measures. Many governments have sought to sign bi- or multilateral free trade agreements constituting an agreement to mutually lower these barriers. The U.S. currently has 14 agreements with 20 countries, according to the Commerce Department.

Republicans have often argued for such measures as an extension of their free market economic goals, and Democratic presidents such as Bill Clinton and Barack Obama have joined in supporting major international trade deals. Opposition has tended to come from the pro-labor quarters of American liberalism, as unions worry their constituencies won't be able to compete against cheap foreign workers.

This has played out prominently in recent debates over a proposed Pacific trade agreement, known as the Trans-Pacific Partnership (TPP), which would include 11 countries besides the United States — Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam.

Some liberal politicians, including U.S. Sen. Elizabeth Warren of Massachusetts and presidential hopeful U.S. Sen. Bernie Sanders, argue that trade agreements frequently do more harm than good for America because they favor the well-being of businesses over workers — both in America and in poorer trading partners.

"These treaties have forced American workers to compete against desperate and low-wage labor around the world. The result has been massive job losses in the United States and the shutting down of tens of thousands of factories," a statement on Sanders' Senate webpage said.

"These corporately backed trade agreements have significantly contributed to the race to the bottom, the collapse of the American middle class and increased wealth and income inequality," he adds.

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Those critical of recent trade agreements also argue that labor standards are not enforced in other countries, so the U.S. is effectively rewarding those businesses abusing their workers.

Those are arguments against free trade when it is practiced perfectly, with both countries firmly dedicated to maintaining low barriers to trade, but many argue these agreements are bad for the U.S. because its partners often seek to subvert the relationship.

The most common charges of "cheating" on free trade involve a country's keeping its currency artificially low (so its products are cheaper, and therefore more competitive, in the U.S.), subsidizing its domestic firms so they have better margins, or even levying protective tariffs without regard for prior agreements.

If the U.S. leaves its markets open to unfairly cheap foreign products while its own goods are stymied abroad, then the job-killing concerns about free trade are all the more pressing,

"Trade deals are absolutely killing our country — the devaluations of their currencies by China and Japan and many, many other countries, and we don't do it because we don't play the game," GOP front-runner Donald Trump said at a Thursday night debate, reiterating his call to employ threats of retaliatory tariffs. "And the only way we're going to be able to do it is we're going to have to do taxes unless they behave."