Anheuser-Busch InBev NV’s bid to buy Grupo Modelo SAB may face an obstacle unforeseen by the parties when the Justice Department sued to block the deal: assignment of the case to Washington’s slowest federal judge.

U.S. District Judge Richard Roberts, who was randomly picked to handle antitrust litigation over the $20.1 billion deal, carries the largest backlog of unfinished cases among his Washington colleagues, according to federal court data.

Last year, Roberts had 73 motions pending for more than six months and more than 50 cases pending more than three years. In one 2003 case involving $190,000 of insurance payments, he took more than eight years to rule on a motion to dismiss.

“It helps the Justice Department,” Michael Carrier, an antitrust law professor at Rutgers University, said in an interview. “If they have a judge that is this slow — and obviously this is something they could not have prepared for — there’s no doubting this will help the government.”

Delays in the AB InBev case could derail the deal if the litigation pushes past the termination date called for in the sale agreement. The parties have until Dec. 30 to complete the transaction, and may extend it another 90 days if it’s in court. After that, Modelo may be able to walk away with the $650 million breakup fee.

Talks between AB InBev, the world’s biggest brewer, and the Justice Department were still proceeding when the suit was filed on Jan. 31, people familiar with the matter have said.

If the case does continue in court, Roberts must independently assess the government’s claim that the combination of the largest- and third-largest brewers of beer sold in the United States would violate antitrust law and “substantially lessen” competition.

Added pressure to settle?

The selection of Roberts could put additional pressure on the company to settle rather than test the government’s case in court, according to Edward Schwartz, an antitrust partner at Steptoe & Johnson LLP in Washington, whose client once waited six months for a final ruling from the judge on a settlement with the department.

“I could imagine it increasing the motivation to some degree to work something out,” Schwartz, who isn’t involved in the case, said in a phone interview.

Marianne Amssoms, a spokeswoman for Leuven, Belgium-based AB InBev, and Gina Talamona, a Justice Department spokeswoman, declined to comment on Roberts’ assignment to the case.

Roberts, 59, was appointed to the court by President Bill Clinton in 1998. He was previously chief of the criminal section in the Justice Department’s civil rights division.

Roberts declined to be interviewed for this story.

In 2002, Roberts told the publication Legal Times in 2002 after it reported he was consistently the slowest judge on the court: “It’s important to me that we decide cases fairly and correctly, as well as efficiently and quickly. I have been a public servant most of my career, and I take seriously the obligation to resolve disputes fairly and promptly.”

At the end of last week, AB InBev and the Justice Department jointly asked for a scheduling conference on Feb. 15 or as “soon thereafter as the court’s docket permits.”

On Monday, Roberts, saying AB InBev hadn’t responded to the government’s lawsuit yet, denied the request for the session. He didn’t set any new date or deadlines.