As the Wisconsin presidential primary gets underway Tuesday, an important issue on the campaign trail surrounding corporate tax loopholes has received an extra push from the president and the U.S. Department of Treasury.

On Monday, Treasury took further steps to close down a corporate tax dodge known as an inversion, and a related practice known as earnings stripping.

Generally speaking, an inversion is a tactic used by a large companies that allows them to avoid paying corporate taxes by purchasing the stock of an overseas entity and re-incorporating in a more tax-favorable country. Earnings stripping further widens the loophole, essentially letting companies borrow from the overseas parent, and claim the interest for debt payments against their U.S. earnings, further reducing their tax burden.

Changing the corporate tax code has received a good deal of attention during the presidential debates and primaries. All five remaining presidential candidates have mapped out positions on corporate tax loopholes, which many experts say saddle small business owners with an ever-bigger share of taxes in the U.S.

President Obama on Tuesday said Treasury's action was necessary to stem what he called the most insidious tax loophole out there. He said big corporations use inversions to game the rules, which "sticks the rest of us with the tab and makes hardworking Americans feel like the deck is stacked against them."

For his part, Republican front-runner and real estate billionaire Donald Trump has said he's sympathetic to companies that use inversions.

"There is no way you can stop [corporate inversions] really other than lowering the taxes because right now ... it is prohibitive to bring that money in," Trump told Bloomberg News in November. "They'd have to pay so much, they'd have to be fools to bring it in."

Trump has indicated that his solution would be to lower the corporate tax rate in the U.S. He favors a rate of 15 percent, down from its current top rate of about 35 percent. (In countries such as Ireland and Switzerland, where many U.S. businesses have pursued inversions, the top corporate tax rate is around 12 percent.) Republican senator Ted Cruz hasn't said much about inversions, but also favors lowering the top corporate tax rate, to 16 percent.

By contrast, both Democratic presidential candidates have taken tough stances against inversions and earnings stripping. Former Secretary of State Hillary Clinton would make it harder to qualify as a foreign company. She would also crack down on earnings stripping, and would impose an exit tax on un-repatriated profits held overseas by companies that use inversions.

Vermont Senator Bernie Sanders, meanwhile, would end inversions altogether.

"My message to these corporate deserters is simple: You can't be an American company only when you want corporate welfare from American taxpayers or you want lucrative contracts from the federal government," Sanders said in a statement in January. "If you want the advantages of being an American company then you can't run away from America to avoid paying taxes."

Small business groups have generally favored closing the loophole as well. John Arensmeyer, the chief executive of the Small Business Majority, says inversions put an unfair burden on small businesses, which continue to pay their share of taxes in the U.S. According to SBM's research, 75 percent of small business owners say their companies are harmed when big businesses use inversions, and 90 percent say they favor closing the loophole.

Despite the hoopla around corporate inversions, it does not seem that all that many companies have actually taken advantage of them. By some estimates, there have been fewer than 80 since 1982, although they have occurred with greater frequency in recent years. About 40 percent of inversions since that time have taken place over the last decade. Nevertheless, the Treasury Department estimates it would get back about $20 billion over the course of a decade by closing the loophole.

Pharmaceutical company Pfizer is the largest company to undertake an inversion in recent months, with a $150 billion takeover offer for Ireland-based Allergan. Pfizer reportedly has sought to avoid paying $35 billion in taxes through the acquisition. The Treasury Department's announcement could complicate that deal, according to some market observers.