But it appears that Mr. Espada may have been driven to make a deal to return as majority leader out of fear of being marginalized, because a separate Democratic faction was moving to establish a power-sharing deal with the Republicans.

Indeed, the Democrats have become increasingly polarized, often along racial lines. Mr. Espada and other Hispanic senators have pushed for more influence from Mr. Smith and Mr. Sampson, who are black.

Separately, the faction of seven white Democrats, led by Senator Jeffrey D. Klein of the Bronx, that had sought the power-sharing deal with the Republicans is especially uneasy with Mr. Espada, who faces investigations related to nonprofit health clinics he runs, his campaign finance practices and whether his primary residence is in the Bronx. Any arrangement they reached with Republicans would probably have pushed Mr. Espada aside.

Faced with that possibility, Mr. Espada returned to the Democrats in exchange for a job whose power beyond its title is difficult to discern. The titles of Senate president and majority leader have traditionally been combined; the president is vested with special powers in the state’s Constitution, and the majority leader is not.

As majority leader, Mr. Espada will receive a bonus on top of his regular legislator’s salary.

Senator Hiram Monserrate, a Queens Democrat who initially sided with the Republicans along with Mr. Espada, played a key role in persuading his colleagues to allow Mr. Espada to return.

Dean G. Skelos, the leader of the Senate Republicans, speculated that the Democratic caucus would break apart again.

“This is my prediction,” Mr. Skelos said at his own news conference, his caucus surrounding him. “Within a few months, maybe six months, there is going to be so much discord within that conference that we’re going to be running the Senate, all right?”