The future of an obscure but far-reaching federal agency has emerged as an early litmus test for Republican presidential contenders, who are being forced to pick sides between the GOP’s powerful business interests.

The Export-Import Bank, which helps underwrite loans for American products sold abroad, is under fierce assault as “corporate welfare” by a political network backed by the billionaires Charles and David Koch and other conservative donors. On the other side, major corporations such as Boeing and top business trade groups such as the U.S. Chamber of Commerce are working frantically to save the bank.

The intense lobbying fight presents one of the greatest threats to the institution since it was created as part of the New Deal in 1934.

“You have something that was a matter of course for many, many years that is now a matter of serious discussion and debate within the Republican caucus,” said Tim Phillips, president of Americans for Prosperity, the Koch-backed conservative advocacy group that is mobilizing its activists against the bank.

Opponents acknowledge, however, that they still face an uphill climb in preventing the reauthorization of the bank, which provides billions of dollars annually in loan guarantees and other financial support for U.S. exports.

The bank’s fate has already surfaced as a key issue for the crop of likely Republican presidential candidates as they work to impress the party’s conservative base. Most of the GOP field has rushed to call for an end to the institution, including former Florida governor Jeb Bush, Wisconsin Gov. Scott Walker, Sen. Marco Rubio of Florida, Sen. Rand Paul of Kentucky, Sen. Ted Cruz of Texas, Louisiana Gov. Bobby Jindal and Indiana Gov. Mike Pence.

Bush’s stance is particularly striking: He was affiliated with a Florida company that marketed water pumps to Nigeria as part of a deal that received $74 million in Export-Import Bank financial support.

As a Milwaukee County executive, Walker complained that the bank was blocking financing for a coal-fired power plant in India that wanted to order equipment from a Wisconsin company. The experience convinced him the bank could be manipulated for political purposes, aides said.

“Governor Walker does not believe it should be reauthorized,” AshLee Strong, a spokeswoman for Walker’s political committee, said in a statement.

While past campaigns against the bank have fizzled, its opponents this time include top Republicans such as Rep. Jeb Hensarling of Texas, chairman of the House Financial Services Committee. On Thursday, Ways and Means Committee Chairman Paul Ryan of Wisconsin reiterated his support for letting the bank’s charter expire June 30.

“Just let it go,” Ryan, the 2012 vice presidential nominee, told reporters. “Absolutely. Most of the benefits go to a few very large companies. I see it as crony capitalism.”

But bank officials say that its demise would hurt American workers. The bank says it provided $20.5 billion in financial support for goods manufactured in the United States and exported to other countries during fiscal 2014.

“Since 2009, Ex-Im Bank has created or sustained 1.3 million American jobs — jobs that otherwise would have been created or sustained in China, South Korea, France or some other competitor nation with its own version of the Ex-Im Bank,” bank spokesman Matt Bevens said.

Supporters also note that the majority of the bank’s transactions support small businesses. But the vast share of the total financing it provides boosts large corporations: In 2014, more than $15 billion — or 75 percent of the money authorized by the bank — went to support exports by companies such as Boeing, Caterpillar and General Electric, according to the bank’s annual report.

Tea-party-aligned groups such as the Club for Growth, Heritage Action and FreedomWorks began campaigning against the bank several years ago, describing it as an example of “crony capitalism” that boosts politically connected corporations.

On Capitol Hill, the debate has created a split among Republicans on the House Financial Services committee. The chairman, Hensarling, has made the bank the central focus of his reform agenda, hosting private meetings with GOP colleagues and sending missives to colleagues describing the institution as “hurting U.S. competitiveness” while “assisting foreign companies and governments.”

Now the influential Koch-backed network is stepping up its opposition, rolling out parallel campaigns this week that call for lawmakers to let the bank’s authorization expire.

“True supporters of free enterprise must oppose corporate welfare of any kind,” Marc Short, president of Freedom Partners Chamber of Commerce, the network’s main financial arm, wrote to members of Congress on Thursday.

The lobbying effort is one of the network’s first major public initiatives since the Kochs and other donors pledged earlier this year to spend $889 million in the run-up to the 2016 elections.

Koch Industries, the Wichita-based diversified manufacturing company run by the Koch brothers, is lobbying against the bank’s renewal — even though its subsidiaries have benefited from its financing in the past. For example, overseas customers of Georgia Pacific, the Koch-owned paper products company, received loan guarantees from the bank.

“We’ve lobbied against corporate welfare and will continue to vocally do so,” said Philip Ellender, the company’s president of government affairs. “Koch has never lobbied in favor of Ex-Im, but we will not place ourselves at a competitive disadvantage by not participating in programs that are in place.”

The campaign allows the Koch-backed network — which has operated as a powerful GOP ally — to position itself as independent from Republican leadership.

As part of its “Break the Bank” campaign, AFP is showering primary voters in the districts of 93 House Republicans with mailers and phone calls, urging them to call their lawmakers. This week, roughly 400,000 Republican voters received mailers with the headline “URGENT TAXPAYER UPDATE.”

“The Export-Import Bank has given billions in risky loans to politically connected foreign companies in China, Russia, Venezuela and Saudi Arabia — all backed by your taxpayer dollars,” the mailer states.

The organization is also mobilizing activists to visit lawmakers at the U.S. Capitol and in their district offices.

For its part, Freedom Partners rolled out a six-figure online ad campaign this week called “Ex-Im Exposed.” The group also issued a point-by-point fact-check of the Chamber of Commerce’s arguments that the bank helps small businesses and saves taxpayers money.

The Chamber shot back this week with a rebuttal, arguing that if the bank fails, U.S. companies will lose ground to foreign companies backed by their own export credit agencies.

“International business is conducted in the real world, which doesn’t always align with the preferences of those of us who support free enterprise and free markets,” wrote Christopher W. Wenk, the group’s executive director of international policy.

This month, the bank’s president, Fred P. Hochberg, visited Oklahoma to shore up support in the heart of the oil and gas industry, part of a grass-roots strategy deployed by bank allies to counter the Kochs.

Jay Timmons, president and chief executive of the National Association of Manufacturers, warned in a letter to members of Congress Thursday that manufacturers “are at risk of losing billions of dollars of sales and global customers.”

“This isn’t a theoretical argument,” he added, “it’s the reality that will be directly caused by politics that further no national interest.”

Alice Crites and Mike DeBonis contributed to this report.