WASHINGTON — Rudolph W. Giuliani, facing a flood of questions about whether his business dealings should disqualify him from being named President-elect Donald J. Trump’s secretary of state, on Tuesday defended his lucrative 15 years in the private sector as a credential for the job.

“I have friends all over the world,” Mr. Giuliani, the former New York mayor, said in an interview. “This is not a new thing for me. When you become the mayor, you become interested in foreign policy. When I left, my major work was legal and security around the world.”

As secretary of state, Mr. Giuliani, a loyal, often ferocious backer of Mr. Trump’s candidacy, would make fighting Islamist terrorism the centerpiece of the incoming administration’s foreign policy. He vaulted to national prominence because of his leadership after the Sept. 11, 2001, terrorist attacks, and he still views foreign policy through the prism of that day.

But Mr. Giuliani’s business ties are a major red flag. He built a lucrative consulting and speechmaking career after leaving City Hall. His firm, Giuliani Partners, has had contracts with the government of Qatar and the Canadian company that is building the Keystone XL oil pipeline, and Mr. Giuliani has given paid speeches to a shadowy Iranian opposition group that until 2012 was on the State Department’s list of foreign terrorist organizations.