For a native New Yorker mounting a first bid for national office, Rudolph W. Giuliani has shown an impressive ability to raise money in Texas, where his Republican presidential campaign collected $2.2 million in the first quarter of the year, far more than any other candidate.

Mr. Giuliani has drawn support from Texans who were notable donors to President Bush, including a former Enron president, Richard D. Kinder, and business executives who direct many of the nation’s oil, gas and energy producers.

And a good part of this success, analysts say, stems from his affiliation with a well-established and politically connected law firm that is based in Houston and bears his name, Bracewell & Giuliani.

That affiliation adds to Mr. Giuliani’s personal wealth but also could pose political risks for him. The firm is perhaps the nation’s most aggressive lobbyist for coal-fired power plants, heavy emitters of air pollutants and carbon dioxide, a gas associated with global warming. Environmentalists say the firm played a significant role in persuading the Bush administration to roll back major provisions of the Clean Air Act.