The resignation on Friday of Julius Genachowskiafter four years as chairman of the Federal Communications Commission again raises a thorny issue forPresident Obama: whether it will be possible to get the F.C.C. or Congress to help him fulfill a campaign promise to guarantee that the Internet remains free and open to businesses and users.

Mr. Genachowski, who said on Friday that he would leave the commission “in the near future,” pushed it in the direction of embracing rules against discrimination by Internet service providers in what content they carry or how fast they transmit it, an issue known as net neutrality.

But he has faced opposition on that front from the federal courts and some telecommunications companies, while consumer advocates have complained that Mr. Genachowski was not bold enough in his efforts.

A law school friend of Mr. Obama and an investor in technology and telecommunication start-ups before coming to the F.C.C., Mr. Genachowski set ambitious goals during his tenure and accomplished some of them, including expanding broadband Internet service and beginning to free up additional airwaves for sale to mobile phone companies.