Three days after Paul Ryan became the presumptive Republican vice presidential candidate, he made a pilgrimage on Tuesday to the Las Vegas gambling palace of Sheldon Adelson, the casino tycoon who is spending more than any other donor to try to send Mr. Ryan and Mitt Romney to the White House. No reporters were allowed, perhaps because the campaign didn’t want them asking uncomfortable questions about the multiple federal investigations into the company behind Mr. Adelson’s wealth.

Those questions, though, aren’t going away, and neither are the ones about the judgment of Mr. Romney and Mr. Ryan in drawing ever closer to a man whose business background should lead them to back away — fast. By not repudiating Mr. Adelson’s vow to spend as much as $100 million on their behalf, the two candidates seem more eager to keep the “super PAC” dollars flowing than to preserve the integrity of their campaign.

The issues swirling about Mr. Adelson’s business practices are not new and can hardly come as a surprise to the Romney campaign. Last year, his company, the Las Vegas Sands Corporation, announced that it was under investigation by the Justice Department and the Securities and Exchange Commission for possible violations of the Foreign Corrupt Practices Act — specifically, that it bribed Chinese officials for help in expanding its casino empire in Macau. Later, the F.B.I. became involved, and even Chinese regulators looked askance at the company’s conduct, fining it $1.6 million for violating foreign exchange rules, The Times reported on Monday.

Then there’s an unrelated investigation by the United States attorney’s office in Los Angeles into whether the Sands Corporation violated federal money-laundering laws by accepting millions from high-rolling gamblers accused of drug trafficking and embezzlement. The Wall Street Journal reported earlier this month that federal authorities are examining whether the casino should have reported the suspicious funds to the government. Instead, the company accepted $100 million from one of the gamblers and gave him free hotel rooms, plane rides and large lines of credit.