HOUSTON (Reuters) - Democratic White House hopeful John Kerry on Thursday challenged President Bush to prove he had not cut a deal with Saudi Arabia by pressuring OPEC oil-producing countries to lower prices now.

Despite assurances from the Saudi ambassador that his country made no agreement to hold down oil prices just before the Nov. 2. election to boost Bush's chances, Kerry said "the simple truth" was that Americans were getting a bad deal.

"Mr. President, I'm here today to say if there was no deal, no agreement ... then stand up today and jawbone OPEC to lower the price now," Kerry told a rally at the University of Houston to mark Earth Day.

Prince Bandar bin Sultan assured Kerry on Wednesday that there was no deal. The Saudi envoy was responding to allegations by Washington Post journalist Bob Woodward, author of a new book on Bush's preparations for the Iraq war -- "Plan of Attack."

He denied the assertion that the Saudis would try to fine-tune oil prices to prime the U.S. economy for the November election, something they understood would favor Bush. He told reporters there was no "political quid pro quo."

The presumptive Democratic presidential nominee has criticized Bush for not taking a tougher stand against the Organization of Petroleum Exporting Countries and Saudi Arabia in particular, after the cartel decided to cut its oil production quota earlier this month.

"I think we have a right to ask OPEC why they're waiting," Kerry said during a campaign swing through Bush's home state of Texas. "What are they waiting for? If they want to help us, this is the time that we need it ... they could up their production tomorrow and we deserve to have them answer us why they won't do that."

U.S. crude oil prices soared more than $1 a barrel to $36.80 a barrel on Thursday, after the Environmental Protection Agency announced the U.S. government would not change its anti-pollution rule limiting sulfur content in gasoline.

The move means that U.S. gasoline supplies are likely to remain tight because refiners cannot import gasoline with high sulfur content on a temporary basis. Americans already are paying record high prices at the pump.

Kerry pledged openness in his dealings, blasting Bush, a former Texas oilman, and Vice President Dick Cheney, who ran Halliburton, the Texas-based oil services giant, for writing an energy policy in "secret deals ... secret meetings."

"I pledge to you when I am president no deal will be cut and no legislation will be written by polluters in exchange for campaign contributions and support," he said.

The four-term Massachusetts senator, locked in a tight race with Bush for the White House, said earlier this week it would be "outrageous and unacceptable" if the Bush administration and the Saudis reached a secret deal to tie the price of gasoline and fuel supplies to the election.

"I don't know, and nor do you. I don't know if it was a deal. I don't know if it was a secret pledge. I don't know if it was just a friendly conversation among friends. But here is the simple truth: The fact remains that whatever it was, Americans are getting a bad deal today."

Bandar has been the Saudi envoy to the United States for 20 years and is part of the Saudi royal family, which has had a long-standing close relationship with the Bush family.