Sen. Richard Shelby (R-Ala.) speaks at a Senate hearing. (Aaron P. Bernstein/Reuters)

Tensions flared Wednesday over a stalled disaster aid bill, with White House officials venting privately about Senate Appropriations Chairman Richard C. Shelby (R-Ala.) even as Shelby openly questioned whether White House Acting Chief of Staff Mick Mulvaney is playing a constructive role in talks.

“Constructive role? He’s certainly playing a role,” Shelby told reporters at the Capitol when asked about Mulvaney.

The issue that has surfaced between Shelby and the Trump administration in recent days is the Harbor Maintenance Trust Fund, which collects fees from users to pay for upgrades at ports.

Shelby hopes to include language in the disaster aid bill that would make it easier to spend money in the fund, noting that repairs are badly needed at ports that have been affected by recent hurricanes. But White House officials oppose Shelby’s approach, saying he is just looking for a way to spend more money.

The flare-up over the fund underscores the difficulties lawmakers have encountered as they try to make a deal on a long-stalled $17 billion disaster aid bill, which contains money for natural disasters that have occurred nationwide, including wildfires in California, flooding in the Midwest to hurricanes and tornadoes in the South.

Two White House officials, speaking on the condition of anonymity to discuss internal thinking, said Shelby is focusing on the harbor fund to the exclusion of other issues in the disaster aid bill.

The officials also said Shelby is resisting the administration’s attempts to add to the measure emergency funding for the U.S.-Mexico border, an issue that came up at a private lunch Tuesday with GOP senators and Vice President Pence. At the time, Shelby told Pence that adding other matters to the bill could only complicate negotiations.

The issue of the Harbor Maintenance Trust Fund came up in a phone call Tuesday between Shelby and Mulvaney that Shelby described as “candid.”

“I think it’s kind of a fraud on the people; they’re collecting it to maintain the harbors and the ports, and so our infrastructure’s deteriorating everywhere. So I say we’re trying to loosen it up,” Shelby said. “I talked to Mulvaney about it; he doesn’t want to do it,” Shelby added, describing this stance as “mind-boggling.”

John Czwartacki, a spokesman for Mulvaney, said: “We are looking forward to the House and Senate passing a disaster-relief bill to bring aid to those impacted as soon as possible. Other than that, we are not in the habit of commenting on private and deliberative conversations with Members of Congress.”

Much of the dispute over the disaster legislation, which has been pending in one form or another since last year, has focused on a dispute between President Trump and congressional Democrats over aid to Puerto Rico. That issue, too, remains unresolved, but has been joined by a cluster of other matters — namely the harbor fund and the border money — in creating fresh complications for the disaster legislation.

Lawmakers have grown increasingly frustrated about the impasse, which has repeatedly appeared to be nearing a resolution only to encounter new roadblocks.

On Friday, House Democrats plan to pass their own version of the disaster bill, one that contains hundreds of millions of dollars more for Puerto Rico than Trump or Senate Republicans have supported. The House move appears unlikely to break the stalemate in the Senate, but some lawmakers hope it could set the stage for a final resolution next week.

Meanwhile, the ill will between Shelby and Mulvaney could bode poorly for the numerous budget challenges that loom later this year, including raising the government’s borrowing limit, lifting budget limits and renewing government funding before it expires Sept. 30, when another government shutdown could occur.

“He is the White House. I mean, he works at the White House, he’s the staff director to the president, acting staff director,” Shelby said of Mulvaney, whose title is acting White House chief of staff. “So they’re going to be involved, they’re always involved. But that’s just life up here.”