As Senate Republicans scramble to determine who will succeed Pennsylvania’s Arlen Specter as the top Republican on the Judiciary Committee, Charles E. Grassley , R-Iowa, has some decisions to make — and perhaps even some favors to call in.

In the coming days, panel Republicans must decide who will be ranking member for the rest of the 111th Congress. In the end, they may have to choose whether to install a conservative hard-liner — Alabama’s Jeff Sessions — now or preserve a spot for a more moderate “Old Bull” — Grassley — in the 112th Congress.

For the various factions of the GOP conference, the stakes were raised significantly May 1 when Supreme Court Justice David H. Souter confirmed reports that he plans to retire at the end of the court’s term in June. Whoever steps in for Specter, who defected to the Democrats last week, will probably lead any opposition to President Obama’s first nominee to the high court.

A Republican leadership aide said senators would abide by GOP caucus rules for filling committee posts. Under those rules, panel Republicans will decide among themselves who their ranking member will be for the remainder of the 111th Congress. That choice will go before the full conference, which must ratify the recommendation before it can take effect.

The aide said May 1 that the decision was expected “to happen very swiftly,” but would not specify whether the selection would be made in time for the full conference to act at Tuesday’s caucus lunch.

The line of succession is complicated by GOP term limits, which bar the No. 2 Republican and former chairman — Orrin G. Hatch of Utah — from taking over unless he were to receive a waiver. Grassley, who is next in line, would have to give up his top spot on the Finance Committee, a plum assignment from which he is poised to play a pivotal role in the coming debate over health care. Then comes Minority Whip Jon Kyl of Arizona, who would keep his leadership position rather than a ranking committee slot, followed by Sessions.

Grassley said last week that he preferred to remain at Finance for the remainder of the 111th Congress but might reconsider if something was “going to happen in the interim” that would box him out of moving to the top Judiciary post at the start of the 112th.

That “something” could include Sessions, who some believe might not want to give up the Judiciary post if he gets it and who already has support from conservative members of the panel.

Kyl has said Sessions is “next in line” and is expected to back him. A Senate source said May 1 that another conservative, Oklahoma’s Tom Coburn , is “probably going to support Sessions” as well.

Sessions was attending an aunt’s funeral in Alabama last week and was unavailable for comment.

Nomination of Sessions in 1986

In 1986, President Ronald Reagan selected Sessions for a federal judgeship. His was only the second judicial nomination in 48 years to be killed by the Judiciary Committee, which on a 9-9 vote refused even to let his name go to the floor for a vote. Opponents accused him of “gross insensitivity” on racial issues.

Sessions was quoted then as saying that the Senate on occasion had been insensitive to the rights and reputation of nominees. After joining the committee, he remarked that his presence there, alongside several of the members who voted against him, was a “great irony.”