But today, Mr. Yang, along with tens of millions of dollars in payments the Sands made through him in China, is a focus of a wide-ranging federal investigation into potential bribery of foreign officials and other matters in China and Macau, according to people with direct knowledge of the inquiries.

The investigations are unfolding as Mr. Adelson has become an increasing presence in this year’s presidential election, contributing at least $35 million to Republican groups. On Tuesday, Mitt Romney’s running mate, Representative Paul D. Ryan, is to appear at a fund-raiser at the Sands’s Venetian casino in Las Vegas; Mr. Adelson is likely to attend, a person close to him said.

In the political arena, Mr. Adelson is perhaps best known as a hawkish defender of Israel. But whatever the outcome of the inquiries involving his businesses in China, an examination of those activities suggests a keen interest in Washington’s China policy and highlights the degree to which politics and profits are often intertwined for Mr. Adelson.

The Sands has faced a conundrum in China as a casino company whose fortunes are heavily dependent on its operations in a country where gambling is illegal, except in Macau. The company relies on the good will of Chinese officials, who mete out approvals and have the power to curtail the flow of mainland visitors. As a result, Mr. Adelson has sought to use financial clout and connections to exert political influence at the highest levels of government.

On the front lines of those efforts was Mr. Yang, who was paid $30,000 a month by the company before he was fired in 2009, he said. At times, he acted as Mr. Adelson’s personal guide to the Chinese establishment. Among the dignitaries he took Mr. Adelson to see was Wan Jifei, a leading international trade official whose father had been vice premier. That led to a lunch with other trade officials at the Great Hall of the People on Tiananmen Square.