He has hit the road as a best-selling author, dabbled in presidential politics, tried his hand at overseas security consulting and hopped aboard the corporate speaking circuit.

But after a sometimes bruising and unfailingly colorful decade, Rudolph W. Giuliani is turning his relentless personality and blustery style back to the New York City office that catapulted him to national fame.

With the man he endorsed to succeed him, Michael R. Bloomberg, about to leave office, Mr. Giuliani is determined to play an outsize role in the race to replace him, quietly putting his political muscle and bulging Rolodex behind a former top City Hall deputy who is seriously considering a mayoral run.

Over the last few weeks, Mr. Giuliani and a coterie of former aides have coalesced around the deputy, Joseph J. Lhota, a Bronx-born Republican, with a single-mindedness that borders on fervor, encouraging him to leap into the campaign and talking up his prospects to business and political leaders, at times well beyond the borders of New York. In Ohio, a few days before the presidential election, Mr. Giuliani was overheard extolling Mr. Lhota’s mayoral qualifications backstage at a rally for Mitt Romney.