President Obama has yet to reveal his choice to succeed Attorney General Eric H. Holder Jr., but already the Senate confirmation process has begun its march toward contentiousness.

With Nov. 4 midterm elections potentially tipping the balance in the Senate, some Republicans immediately called for a delay in the hearings and votes on the new attorney general until January, when the possibility of a GOP majority in the Senate might give Republicans almost total control of the outcome.

Sen. Ted Cruz (R-Tex.) issued a political call to arms for conservatives, saying that outgoing senators should not vote on the nominee during the post-election lame-duck session. “Allowing Democratic senators, many of whom will likely have just been defeated at the polls, to confirm Holder’s successor would be an abuse of power that should not be countenanced,” Cruz said in a statement.

Democrats argued that Republicans should step back and allow Obama to select his own cabinet without GOP obstruction.

“This is going to be the first real test, whether it’s in the lame-duck or early in the new year, whether our Republican colleagues are going to continue to obstruct,” Sen. Charles E. Schumer (D-N.Y.) said Thursday in an interview. “Every president deserves to have his attorney general.”

Obama, in remarks Thursday at the White House, gave no indication of when he would announce Holder’s replacement or who that person might be. The resignation announcement plunged the Washington legal community into an urgent parlor game of ranking potential candidates’ chances. Some Democrats suggested that it could be weeks before a nominee is announced, while others said that senior Obama officials have begun informal discussions with potential nominees.

Possible contenders include U.S. Solicitor General Donald B. Verrilli Jr.; former White House counsel Kathryn Ruemmler; Tony West, the former associate attorney general who just stepped down; Loretta E. Lynch, the U.S. attorney for the Eastern District of New York; Preet Bharara, the U.S. attorney for the Southern District of New York; and Jenny Durkan, who is about to step down as the U.S. attorney for the Western District of Washington state.

Some of the president’s supporters say the list represents top-quality choices. Neal Katyal, who served as acting solicitor general of the United States under Obama and is now a partner at Hogan Lovells, said in an interview that in contrast to the beginning of the Obama administration, “Now you have really a deep bench of people who understand what the mission of Department of Justice is and have served at its highest levels.”

“I expect that the White House will nominate someone who has Republican support in the Senate and that the Senate will not delay consideration of the nation’s chief law enforcement official,” Katyal added.

Under normal circumstances, it would take about two months for White House’s selection to be vetted by the FBI and Senate Judiciary Committee and then go through a final confirmation vote by the full Senate. But the election calendar will probably affect that time frame. Holder’s decision to leave on the eve of a midterm election has no precedent in recent history, and the slow-moving confirmation process for Obama’s nominees has been a partisan flashpoint in Washington over the past four years.

Before Obama formally announced Holder’s departure, Republicans were seeking every possible leverage point in the nomination hearings. “Rather than rush a nominee through the Senate in a lame duck session, I hope the President will now take his time to nominate a qualified individual who can start fresh relationships with Congress,” Sen. Charles E. Grassley (Iowa), the top Republican on the Judiciary Committee, said in a statement.

But that could mean a confirmation in a GOP-controlled Senate. Republicans hold 45 seats at the moment and are all but certain to gain at least three more in November; a six-seat gain would put them in the majority and in control of the legislative committees, making Grassley chairman of the judiciary panel.

Because only a simple majority vote is required to clear the Senate’s notorious procedural hurdles and win confirmation, Democrats can almost guarantee confirmation of any Obama nominee if the confirmation votes are held during the lame-duck session after the November elections. The judiciary panel’s current chairman, Sen. Patrick J. Leahy (D-Vt.), made clear that he prefers a quick set of hearings and votes.

“I’ve also talked with the White House and hope that they would be able to decide on a nomination soon,” Leahy said on MSNBC’s “Andrea Mitchell Reports.” “Definitely, we should have confirmation hearings as quickly as possible in the Senate.”

When it came to filling the attorney general slot late in George W. Bush’s second term, Leahy moved quickly and confirmed Michael Mukasey in just eight weeks in late 2007.

The pace may depend on whether the new nominee has recently gone through the confirmation process, in which case the vetting of his or her background will be fresh in the files for the FBI and Judiciary Committee staff. Verrilli, Durkan, Lynch and Bharara have all already been confirmed by the Senate for their legal posts during the Obama administration.

Ruemmler, a close confidant of the president, attracted national attention in her mid-30s when she was one of the three lead federal prosecutors in the case against Enron executives Kenneth L. Lay and Jeffrey K. Skilling, who were convicted on securities and wire-fraud charges. Highly trusted by Obama and his top deputies for her sharp intellect and fierce defense of the executive branch’s powers, she has also sparred with Republicans such as Rep. Darrell Issa (Calif.) over her unwillingness to turn over internal White House documents.

Verrilli, 57, is the nation’s 46th solicitor general, the lawyer charged with representing the government before the Supreme Court. He is a graduate of Yale and Columbia Law School and was a clerk to Supreme Court Justice William J. Brennan Jr. Obama nominated him in January 2011 to replace Elena Kagan, whom the president had nominated to the Supreme Court.

Verrilli was confirmed in a 72-16 vote and supported by 26 Republicans. He previously was co-leader of the Supreme Court practice at Jenner & Block, with an emphasis on First Amendment, telecommunications and copyright law.

Meanwhile, gay rights groups such as the Human Rights Campaign have argued for some time that Obama should nominate an openly LGBT Cabinet member.

They are pushing the president to consider highly qualified candidates such as Durkan, the first openly gay U.S. attorney to be appointed by a president and confirmed by the Senate. Durkan’s name was also floated during last year’s search for Homeland Security secretary. And while Bharara, the U.S. attorney for the prominent Southern District of New York, has attracted significant media attention during his tenure, Democrats said a more serious contender for Holder’s job is the U.S. attorney for the Eastern District of New York, Lynch.

Lynch, who is African American, occupied that same post under President Clinton and is well known within the department as the chair of the Attorney General’s Advisory Committee.

Another potential successor, Securities and Exchange Commission Chairman Mary Jo White, won a unanimous confirmation to that post last year. She also served during the Clinton administration in the prestigious post of U.S. attorney for the Southern district of New York.

Correction: An earlier version of this article had a misspelling of Neal Katyal’s name. This version has been corrected.