WASHINGTON (Reuters) - The corporate income tax is a major revenue source for the U.S. government, but it has been shrinking for decades and the three main presidential candidates could not differ more dramatically on what to do about it.

U.S. Republican presidential candidate Donald Trump hands a five-dollar bill back to a supporter after signing it for her following a rally with sportsmen in Walterboro, South Carolina February 17, 2016. REUTERS/Jonathan Ernst

Republican Donald Trump and Democrats Hillary Clinton and Bernie Sanders have starkly contrasting plans, some more detailed than others, for taxing corporations.

The tax paid by companies on their profits accounted for 30 percent of the U.S. federal tax base in the 1950s; last year, it was only 11 percent, Internal Revenue Service data show.

The decline, experts say, has many causes: loopholes that cut corporations’ taxes; a lower corporate tax rate; rising overseas profits that are lightly taxed; and a shift away from incorporation toward partnerships and other business structures that pay no corporate tax.

Despite the dwindling corporate tax base, Trump, the Republicans’ presumptive nominee for the Nov. 8 election, wants to ease the burden on companies by slashing the corporate tax rate to 15 percent from 35 percent. In this regard, he shares the same view as most Republicans.

Trump, a real estate developer, has described his proposals as revenue neutral, saying reduced tax rates would be paid for by eliminating some tax breaks and repatriating corporate cash held overseas.

However, the Tax Policy Center, a centrist tax research group in Washington, said that under Trump’s plan, corporate income tax revenues would fall $1.9 trillion from 2016 to 2026.

A cut on that scale would hugely expand the federal budget deficit, unless severe budget cuts or alternative tax hikes were also made, the center said.

Steven Rosenthal, a Tax Policy Center senior fellow, said Trump’s plan is a standard business-focused approach, but “the drafting is incomplete, so we can’t tell exactly what he’s thinking.”

A Trump campaign spokeswoman pointed to the proposals on the campaign website but declined to comment further.

Republicans generally support corporate income tax cuts because, they say that would help corporations compete globally. In addition, some question the validity of the tax, which they say is passed on to consumers and investors.

“Taxing businesses is the Democrats’ version of manna from heaven: someone else supposedly is paying for the government. But all citizens are hurt through lower growth, higher prices, lower wages,” said Grover Norquist, a Republican tax activist.

“Taxes should be transparent. It should be clear who is paying them. The corporate income tax is designed to hide the tax burden,” said Norquist, president of Americans for Tax Reform, a small-government group that opposes tax increases.

CLINTON, SANDERS

Democrats tend to view the corporate income tax as a fair levy on businesses that benefit from the security, services and infrastructure provided by government. Corporations have been taxed by Washington since 1909.

Clinton, a former secretary of state and the front-runner for the Democratic nomination, has not promised a corporate tax cut.

Like Trump, she has called for closing loopholes that corporations use to avoid taxes. But unlike Trump, her plan would raise corporate tax revenues by $136 billion over 10 years, the Tax Policy Center said.

Clinton spokesman Jesse Ferguson said the candidate’s proposals “go further to prevent the erosion of the corporate tax base by ensuring corporations pay their fair share.” But the Clinton campaign has yet to release its full plan.

Sanders, a U.S. senator from Vermont challenging Clinton for the Democratic nomination, would make more drastic changes and boost the corporate tax take by $1 trillion in the same decade, mostly by raising taxes on overseas profits, according to the Tax Policy Center.

“Virtually all of the revenue raised by Senator Sanders’ corporate tax plan would come from large, multinational corporations that have shifted their profits offshore to avoid taxes,” said Sanders policy adviser Warren Gunnels.

FAIR SHARE?

The question of whether businesses pay their fair share of taxes strikes a chord with Americans on the campaign trail. An April Gallup poll found 67 percent of U.S. adults said corporations do not pay enough tax.

Since the mid-1950s, income tax receipts from individuals have held steady at 40-50 percent of federal revenues, while payroll taxes, deducted from the paychecks of wage-earners, have soared to 34 percent last year from 5-10 percent in the 1950s.

Meanwhile, Congress has steadily reduced the corporate tax rate, now 35 percent. That is down from 46 percent in 1986, and 52 percent in 1956. Over that period, corporations have also carved out various loopholes that let them avoid paying the full rate.

The corporate tax applies only to incorporated businesses, while other business structures have been gaining in popularity. In 2003, the share of U.S. business income generated by incorporated companies fell below the share made by partnerships and other kinds of companies that pay no corporate tax.

Whatever candidates may say on the campaign trail, no changes can occur without agreement by Congress. And despite years of talk about the importance of tax reform, neither President Barack Obama nor lawmakers on Capitol Hill have been able to agree on a reform agenda that many say is needed to boost the economy and bring sanity to a tax code of Byzantine complexity.