MADRID — The legal troubles of the American casino mogul Sheldon G. Adelson in the United States and Macau are fueling opposition to his plans to build a casino and leisure resort in Madrid or Barcelona.

On Wednesday, a Catalan environmental party, ICV-EUiA, called on Mr. Adelson to appear before the Catalonian Parliament to explain why some of his company’s activities in Macau have run into legal problems in the United States.

Mr. Adelson’s company, Las Vegas Sands, is being investigated by the U.S. authorities on suspicion of violating anti-bribery laws linked to its expansion in Macau, the Chinese gambling capital and a major source of income for the Sands. The company has denied wrongdoing and has said the investigation stems from the accusations of a disgruntled former employee. The Chinese authorities have also been investigating some of the company’s activities.

Jaume Bosch, a lawmaker from the Catalan Green Party, expressed concern that the Spanish authorities had recently welcomed Mr. Adelson, a billionaire, “like a head of state, while the most powerful countries in the world have placed him under suspicion.”

In the United States, Mr. Adelson’s preference for working below the radar has been tested this year as he has emerged as the largest donor to the Republican Party in the presidential campaign, in which he has pledged to spend up to $100 million to defeat President Barack Obama. This week, just days after the presumptive Republican nominee, Mitt Romney, named him as his running mate, Representative Paul D. Ryan traveled to Las Vegas to meet Mr. Adelson for what an aide to Mr. Romney called “a finance event.”

In recent months, plans by Las Vegas Sands to build a giant casino complex in Spain have created a bidding battle between the two biggest cities in the country. Officials are eager to attract development while Spain is in the midst of its second recession in three years, with record unemployment of almost 25 percent. The government is struggling to convince investors that the country will not sink deeper into crisis and require a full European bailout.

Mr. Adelson and other Sands executives have made several visits to possible building sites in Madrid and Barcelona. The two cities also recently sent delegations to Las Vegas in a bid to win over company executives. The Sands wants the complex, which it plans to call EuroVegas, to have 12 hotels with a total of 36,000 rooms, six casinos with 18,000 slot machines and three golf courses.

Esperanza Aguirre, the head of Madrid’s regional government, has urged the national government to meet concessions requested by Mr. Adelson, including some tax exemptions, as well as an easing of Spanish restrictions on smoking in public spaces. Ms. Aguirre’s request, however, has led to questions over whether granting special treatment to the Sands would open the door to similar concessions for other investors.

While a final decision was initially expected before the summer, the Sands has pushed back any announcement until September. The Sands still hopes to start construction in mid-2013 and complete the complex within 10 years, according to Michael A. Leven, the company’s president. Mr. Leven said in June at a news conference in Barcelona that the decision to build a casino complex in Spain should not be compared to “a soccer match between Madrid and Barcelona.”

Still, some Spanish politicians are acutely aware that the longstanding rivalry between the two cities is helping the politically savvy Mr. Adelson.

“It has worked great for him to have two cities competing so fiercely, with everybody trying to make the maximum efforts to meet his wishes,” Xavier Trias, the mayor of Barcelona, said this year. “But if he thinks the European casino model can become the same as in China or Vegas, I think that he is wrong because there are laws here that really cannot and shouldn’t be changed.”

As an example, Mr. Trias said, “In Vegas you can cross a casino to a hotel room with children, but that isn’t possible here.”

Some civic groups oppose plans for EuroVegas on environmental grounds. Other opponents say they fear that by promoting gambling, Spain might spur criminal activities like prostitution and money laundering.

“Our politicians are presenting this as a project that will save us from the economic crisis, but they’re ignoring several reasons why this goes against what this country should push for,” said Sonia Pomares, a member of Aturem EuroVegas, or Stop EuroVegas, a protest association formed this year in Catalonia.