LAS VEGAS, Nevada — The Obama campaign warned Tuesday that Mitt Romney would have to confront questions on whether his offshore holdings were set up to avoid taxes, during Wednesday's presidential debate.

On the eve of the clash in Denver between the Republican and President Barack Obama, a New York Times report suggested tax arrangements of Bain Capital, the company Romney' founded, may have helped enhance his wealth.

"This raises a lot of questions that the Romney campaign should have to answer, he should have to answer tomorrow evening," said Obama spokeswoman Jen Psaki.

"We look forward to hearing what he has to say."

The Times examined thousands of pages of previously unreported documents from Bain Capital, a private equity firm, relating to dozens of offshore holdings in the Cayman Islands.

The paper said the arrangements in some cases allowed the former venture capitalist's retirement account to avoid taxes and may have cut his personal income tax bills.

Psaki said that the Obama campaign was not suggesting that Romney broke the law but suggested Americans deserved to know more about his financial arrangements.

"This is an issue where I think he has been telling people for months that he didn't financially benefit. It is clear that some of the steps his company (took), he did."

Psaki said that Romney could clear up questions about his tax record by offering more than the two years of tax returns he has publicly released.

"It raises the question for people who will be watching at home tomorrow night. Why are they paying a higher rate than one of the presidential candidates?"

Romney senior advisor Michele Davis said that the assets of Romney and his wife Ann were held in a blind trust.

"They have no role in investment decisions, which are made by the trustee. The Romneys have scrupulously followed the tax laws and have paid 100 percent of what they owed."

Bain Capital also issued a statement to the Times, saying that it, like other global asset managers used "widely accepted, fully legal and recognized structures so that investors may receive predictable tax treatment on investment gains for their constituents."

The Obama campaign has repeatedly hammered Romney over his taxes, saying that they are an example of how wealthy Americans can get preferential treatment which is not easily accessible by the middle class.

The Republican challenger insists his arrangements are entirely above board, but that he does not want his opponents to pick over the fine detail of a decade of family finances, despite fierce political pressure.

His return for 2011 shows he paid $1.9 million in taxes on an income of $13.6 million, an effective rate of 14.1 percent.

The rate was so low because investment income is taxed at a lower rate than ordinary salaried income.