By Brent Kendall And Shalini Ramachandran 

Justice Department antitrust enforcers doubt that their concerns about Comcast Corp.'s planned acquisition of Time Warner Cable Inc. could be resolved by promises about how the cable giant would conduct business after the merger, according to people familiar with the matter.

Such commitments, known as behavioral remedies, aren't usually the preferred approach for the Justice Department when it seeks to address mergers it views as problematic. It has accepted them in some circumstances, however, including when it allowed Comcast to acquire control of NBCUniversal in 2011.

It isn't known what promises Comcast might be prepared to offer, but when the cable giant announced the Time Warner Cable deal in February 2014, it said it was prepared to extend certain commitments it made in the NBCUniversal transaction to the current deal.

To ease concerns about the NBCUniversal deal--which involved accumulation of a copious TV and movie content--Comcast agreed to an array of conduct conditions with the department and the Federal Communications Commission.

Those included promises not to discriminate against online video distributors or retaliate against other networks, cable programmers or studios for licensing content to Comcast's competitors. The company also agreed to follow open-Internet rules.

People familiar with the government's 14-month review of Comcast's proposed $45 billion acquisition of Time Warner Cable say similar types of promises would be a tougher sell with enforcers this time around, at least in part because there are questions about whether the earlier set of conditions have worked as intended.

Comcast and Time Warner Cable are expected to meet with Justice officials Wednesday. The meeting is an opportunity for them to discuss potential remedies to address government concerns that the deal would give Comcast too much power over Internet broadband and too much leverage over TV channel owners and competitors who offer video programming online.

It isn't known whether Comcast and Time Warner Cable can offer concessions the government would find satisfactory.

The Wall Street Journal reported Saturday that the Justice Department and the FCC also have been examining whether Comcast has fully complied with its earlier commitments, such as whether it observed its pledge to remain a silent owner with no sway over management decisions at its part-owned streaming site Hulu.

Comcast spokeswoman Sena Fitzmaurice said in a written statement that "in addition to the over 150 conditions from the FCC's NBCUniversal Order, we've complied with the DOJ consent decree fully." She said Comcast has exceeded some conditions, such as by extending its low-income broadband program indefinitely.

Ms. Fitzmaurice said a Comcast-Time Warner Cable deal promises a better video and broadband experience for customers, noting that Comcast offers twice as much video-on-demand as Time Warner Cable and 25% faster Internet speeds. "These benefits all come with no reduction in competition for consumers," she said.

Antitrust officials usually favor structural deal fixes to mergers they find problematic rather than behavioral fixes.

With structural fixes, companies pledge to sell off some of their existing businesses or other assets to rivals to preserve competition that might otherwise have been lost because of the merger.

Such divestitures can be a way to address antitrust concerns without triggering a government lawsuit to block the deal.

Comcast, however, may not have a lot of wiggle room to offer additional divestitures before getting to the point where the Time Warner Cable acquisition becomes less attractive.

The companies already have agreed to deals with Charter Communications Inc. to sell or spin off systems serving 3.9 million customers if the merger goes through.

A Justice Department spokesman declined to comment.

Remedies that focus on business conduct instead of divestitures can be messy to enforce and require government officials to predict how a merged company might act in future market conditions.

Behavioral remedies also may require the Justice Department to maintain a continuing role as a watchdog, making it more of a regulator than a law enforcement agency.

The department's current antitrust chief, Bill Baer, has a reputation for disfavoring the use of behavioral remedies to fix mergers when such an approach is avoidable.

Mr. Baer is recused in the Comcast matter because in private practice he represented NBCUniversal during the earlier transaction, but his views have set the tone for the department.

"Remedies in our horizontal merger and Sherman Act cases should maximize competition--existing or new--and minimize the need for ongoing regulatory involvement," Mr. Baer said in a February speech.

At an American Bar Association antitrust conference last week, Deputy Assistant Attorney General David Gelfand said the department had a "strong preference" for using structural remedies over conduct. He didn't specifically address the department's review of the Comcast deal.

Separately, six senators, led by Minnesota Democrat Al Franken, sent a letter to the FCC and Justice Department calling on the agencies to block the deal. Lawmakers have no direct say in whether a merger receives approval. Sen. Franken has been a vocal critic of the transaction.

Access Investor Kit for Charter Communications, Inc.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US16117M3051

Access Investor Kit for Comcast Corp.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US20030N1019

Access Investor Kit for Comcast Corp.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US20030N2009

Access Investor Kit for Time Warner Cable, Inc.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US88732J2078

Subscribe to WSJ: http://online.wsj.com?mod=djnwires