WASHINGTON — In the world of big-dollar political donors, Imaad Zuberi is notable less for the scale of his giving than for its baldly transactional nature. A supporter of President Barack Obama and then Hillary Clinton’s 2016 campaign who frequently posted pictures of himself alongside high-profile politicians, Mr. Zuberi, a California venture capitalist, abruptly pivoted after Donald J. Trump’s victory.

Telling friends he needed to act quickly to balance out his political connections if he hoped to maintain access, he donated more than $1.1 million to committees associated with Mr. Trump and the Republican Party in the three months after the 2016 election.

It seemed to work. Mr. Zuberi scored coveted invitations to a pair of black-tie dinners celebrating Mr. Trump’s inauguration. In the process, he posted photos of himself with the president, as well as Mr. Trump’s first chief of staff, Reince Priebus, and Mr. Trump’s picks for Treasury secretary, Steven Mnuchin; housing and urban development secretary, Ben Carson; and defense secretary, Jim Mattis.

But the biggest donation of his postelection flurry — $900,000 paid by Mr. Zuberi’s California firm, Avenue Ventures, to Mr. Trump’s inaugural committee — is now being scrutinized by federal prosecutors in the Southern District of New York as part of what appears to be an escalating investigation into the inauguration and its financing.