It's been a tough few days for the U.S. Export-Import Bank. With conservative opposition to the export credit agency long since mounting, this weekend the new House Majority Leader Kevin McCarthy of California said on Fox News Sunday that he supports allowing the bank's charter to expire in September. Then, on Monday, the Wall Street Journal reported that four Ex-Im Bank officials are facing investigations for allegedly accepting improper gifts and kickbacks — a revelation that has only fueled the debate over the future of the 80-year-old federal agency.

The debate is complicated, and it's not easy to tell if the Ex-Im Bank doles out corporate welfare, as its critics say, or offers a valuable safeguard for American businesses, as its supporters would have it. Here's a breakdown of  everything you need to know about the once-obscure agency at the center of Washington's latest skirmish over the legacy of the New Deal:

What is the Ex-Im Bank?

It's both a bank and a federal agency, chartered and in some respects controlled by Congress. It provides guarantees, direct loans, and other financing to foreign buyers in an effort to help U.S. companies sell goods and services abroad.

Founded by Franklin D. Roosevelt's executive order in 1934, the Ex-Im Bank was originally conceived as a New Deal program. Its first transaction was a $3.8 million loan to Cuba to buy U.S. silver ingots. Today, thousands of transactions and billions of dollars later, the Ex-Im Bank often helps corporations make nine-figure deals, frequently in developing markets in Asia and Latin America. 

How does this financing work?

Let's walk through a typical transaction. In 2012, flydubai, an airline owned by the government of Dubai, wanted to buy Boeing 737-800 aircraft from Chicago-based Boeing Co. To finance such a large purchase, a private bank would likely charge a high interest rate on the loan, potentially souring the deal for flydubai. 

The state-run company turned to the U.S. Ex-Im Bank for help. In October 2012, the Ex-Im Bank issued flydubai a $117.5 million loan guarantee so that the airline could finance a 12-year loan through the Private Export Funding Corp. If flydubai defaulted on the loan, Ex-Im Bank would repay the Private Export Funding Corp. — an arrangement that allowed flydubai to pay a comparatively low interest rate and enabled Boeing to secure the deal.

That sounds like a win-win situation. Why are people so upset about it? 

Put simply, the Ex-Im Bank's opponents are concerned about the type of businesses are benefiting from the agency's support. For the past few years, a number of conservative groups and politicians have spoken out against the agency saying it has grown too cozy with big business while interfering in a realm that could be better run by private enterprise. 

Tea party conservatives mounted their first significant attack against the Ex-Im Bank in 2012, the last time the agency was up for reauthorization. Although that effort to close it down was handily defeated, the charter's renewal garnered a surprisingly high 93 nay votes in the House.

After the Ex-Im Bank's 2012 reauthorization, the Club for Growth  one of several conservative groups that have voiced opposition to the agency  campaigned to block the renomination of Fred Hochberg, the agency's head. At the time, the Club for Growth's president, Chris Chocola, called the agency "a slush fund for market-distorting subsidies that pick winners and losers in the private sector."

Now, with the agency up for authorization once more, the bank's opponents and defenders are again coming out swinging. On Wednesday morning, the House Committee on Financial Services is holding a hearing called "Examining Reauthorization of the Export-Import Bank: Corporate Necessity or Corporate Welfare?". 

Who's fighting to keep it alive?

The Ex-Im Bank is backed by a coalition of strange bedfellows  Democrats, some House Republicans, and businesses big and small. Before he was unexpectedly defeated in his primary earlier this month, former House Majority Leader Eric Cantor (R-Va.) was a staunch supporter of the Ex-Im Bank. With Cantor now replaced by an opponent of the agency, a group of 41 House Republicans signed a letter to House Speaker John Boehner (R-Ohio) and McCarthy on Monday urging them to reauthorize the bank. House Democrats have also launched their own effort to keep the bank running. In the business sector, 865 companies and business organizations signed a letter Monday urging lawmakers to the renew the bank that many consider essential to their business abroad.

If it sounds like the players have switched their usual teams, you're right. The Ex-Im Bank debate has splintered the Republican Party between those concerned about corporate welfare and those committed to upholding traditional allegiances to big business. Meanwhile, Democrats have found themselves allied with large corporations in the fight to keep the Ex-Im Bank alive.

What does the WSJ's report on alleged gifts and kickbacks at the Ex-Im Bank mean for all of this?

All things considered, the Wall Street Journal's report couldn't have come at much of a worse time for the bank. The Journal's report, which cited anonymous sources, detailed investigations into misconduct by four employees, including one who allegedly accepted cash in exchange for helping a Florida business finance a deal exporting construction equipment to Latin America.

In the hours since the Journal's report, the Ex-Im Bank has stayed mum, citing privacy laws. Matt Bevens, a spokesman for the agency, told the Journal that "the Export-Import Bank takes extremely seriously its commitment to taxpayers and its mission to support U.S. jobs."

So, who really benefits from the Ex-Im Bank?

It depends on how you crunch the numbers. The Ex-Im Bank has touted its recent success in reaching small businesses, which accounted for nearly 90 percent of its total authorizations in fiscal-year 2013, a record high. When divvied up by dollar volume, however, the numbers look very different. By that metric, small businesses account for only 19 percent of money spent in fiscal 2013  meaning that medium-size and large business are benefiting from four-fifths of the dollars doled out by the Ex-Im Bank.

Is the Ex-Im Bank worth it for taxpayers?

It depends. The agency borrows and pays interest on funds from the U.S. Treasury, and then returns funds that it earns. In fiscal-year 2013, the Ex-Im Bank backed an estimated $37.4 billion in U.S. export sales by authorizing $27 billion in financing, $1.06 billion of which it returned to the U.S. Treasury.

Opponents of the Ex-Im Bank say it is too costly. They often cite a Congressional Budget Office report issued last month that estimated that if the bank used fair-value accounting methods, it would peg the bank's costs to the American taxpayers at $2 billion over the next decade. 

The agency's proponents, however, argue that even if the Ex-Im Bank is costly, its transactions expose taxpayers to relatively minimal risk. That's because the agency's loans and guarantees have the backing of the collateral of real goods, which have a secured buyer and a set price. The default rate was just 0.237 percent in the last quarter of fiscal-year 2013, according to the Ex-Im Bank's 2013 annual report. Overall, less than 2 percent of all Ex-Im-backed loans have been defaulted on since the agency was established in 1934, according to a 2012 statement from the U.S. Chamber of Commerce to the Committee on Ways and Means.

Is the Ex-Im Bank a purely American concept?

No, actually. A number of other wealthy countries have their own versions of the Ex-Im Bank, designed to serve the same purpose of boosting national exports. They're also more generous than the U.S.: The export credit agencies of Canada, Japan, and China have doled out three, five, and 11 times as much financing, respectively, as the U.S. Ex-Im Bank, according to the Chamber of Commerce's 2012 statement.

This competition from foreign export credit agencies  and the market share they could grab if the U.S. Ex-Im Bank is no longer around to level the playing field  has American businesses worried. "Failure to reauthorize Ex-Im would amount to unilateral disarmament in the face of other nations' aggressive trade finance programs," the U.S. Chamber of Commerce's 2012 statement said.

What will happen if the Ex-Im Bank isn't reauthorized in September?

It wouldn't be too dramatic  at least not immediately. If the Ex-Im Bank's charter doesn't get renewed before Sept. 30, the agency won't immediately close its doors. While it would no longer be able to accept new applications for credit or set into motion new loans, it would still have to oversee its existing authorizations and transactions, meaning the bank could stay open for months before it officially shuts down.

Still, the closure of the Ex-Im Bank could have significant political and economic ramifications. Its shutdown would mark a clear and unprecedented legislative victory for the tea party, and it could also cause ripples in international trade. In particular, companies that have benefited from bank loans, including Boeing, Caterpillar, and General Electric, could see an immediate effect. 

Boeing's stock fell after Cantor's loss.