A campaign of negative advertising like we’ve seen this year in Louisiana’s Senate race is far from the textbook ideal of informing the public about issues. Yet, a legitimately debatable difference of opinion about a relatively obscure federal agency has taken on unusual political importance for the candidates.

The agency is the Export-Import Bank, Ex-Im for short. It provides loan guarantees for U.S. businesses selling things abroad.

The reason it is an issue: Republicans tend to be split on renewing its charter.

For many business-oriented Republicans, and moderate Democrats as well, Ex-Im provides a necessary service to export businesses, particularly to compete with the ways our foreign competitors subsidize their companies.

For more conservative Republicans, including insurgent Rob Maness in the Louisiana Senate race, it’s another instrument of “crony capitalism.” Many conservatives want to kill it, exposing a split in the party.

That’s a split that U.S. Sen. Mary Landrieu, D-La., is more than happy to make worse.

Landrieu backs Ex-Im, and headlined — notably, with U.S. Rep. Charles Boustany, R-Lafayette, among others — presentations on Capitol Hill about the Ex-Im benefits for Louisiana businesses.

Critics of the bank reject the notion that it helps small businesses, noting that the bulk of its benefits go to big manufacturers. The Bank of Boeing, they jibe at it — the bank not only provides insurance on international deals like the purchase of airlines, it can also finance the purchases at favorable rates.

So who’s in the political crossfire? Maness’ target, and Landrieu’s, is U.S. Rep. Bill Cassidy.

The Baton Rouge congressman previously voted for reauthorizing the bank. Now that getting rid of it is a tea party enthusiasm, he’s saying the bank must be reformed before it is reauthorized.

Landrieu calls that waffling, and she’s right to a point, but we have a lot of sympathy with Cassidy’s position.

All sorts of business tax breaks are now facing levels of scrutiny that they’ve deserved before, but the political opposition to them was too weak to get a real hearing. When does a pro-business agency become, instead, a big-business boondoggle?

Cassidy’s concerns — even if they’re born of a political ploy — should not be lightly dismissed.

At the same time, we think Landrieu is right on the immediate issue. If the Ex-Im Bank is to be reauthorized this fall, that vote won’t be on the kind of detailed reform package that Cassidy and others — including U.S. Sen. David Vitter, R-La. — want to see. It will likely be an up-or-down vote, and we think that the Congress ought to do it, absent viable alternatives — and those will require at the least a couple of years of deeper thinking on American businesses and their disadvantages in international commerce.

So, we favor reauthorization, and, despite the political hurdles, we think Cassidy ought to, as well.