LAS VEGAS — Four days after his announcement as Mitt Romney’s running mate, Representative Paul D. Ryan was not in Florida talking Medicare with elderly voters or in drought-ridden Iowa talking about a farm bill. He traveled to the Venetian hotel here for a meeting hosted by Sheldon Adelson, the billionaire casino mogul who has pledged to spend as much as $100 million this year to defeat President Obama.

Mr. Adelson has already contributed more money to defeat Mr. Obama than anyone: over $50 million has gone to the 2012 campaign, including $10 million to a “super PAC” backing Mr. Romney and $10 million to Crossroads GPS, which has run millions of dollars of advertisements against Mr. Obama.

In keeping with Mr. Adelson’s penchant for staying below the radar, Romney aides refused to say who attended the meeting with Mr. Ryan, though the location (a private room at one of Mr. Adelson’s hotels) and leaks from the Romney camp left little doubt. And in keeping with laws that prohibit elected officials from explicitly asking donors for super PAC money, aides to Mr. Romney insisted before the event that the meeting was not a fund-raiser.

“It’s a finance event, not a fund-raiser,” an aide told reporters. Asked if people were paying to attend, he repeated, “It’s a finance event, not a fund-raiser.”