WASHINGTON (CN) – President Donald Trump lashed out at congressional Democrats on Monday, following a flurry of comments from key lawmakers indicating that talks to avoid another government shutdown have broken down.

“The Democrats do not want us to detain, or send back, criminal aliens!” Trump tweeted Monday morning. “This is a brand new demand. Crazy!”

A team of congressional negotiators have until Friday to put together a package to fund the government and avert the second government shutdown in as many months. Negotiations began at the end of January, shortly after the White House and Congress struck a short-term deal to end a 35-day shutdown that hinged on $5 billion in funding for a wall along the southern border.

Last week key lawmakers had been optimistic that they would be able to reach an agreement before the deadline.

“The environment’s improved, as far as on the staff level and some of the personal level too,” Senator Richard Shelby, an Alabama Republican and member of the conference committee, told reporters last week. “We’re talking about substance, we’re making overtures both ways.”

Lawmakers also heard from border-security experts in the Trump administration last week about how much money they would like the deal to include for border security and in what form.

But some of the key negotiators on Sunday struck a more pessimistic tone, primarily over Democrats’ demands to limit the number of detention beds for Immigration and Customs Enforcement.

“The Trump admin has been tearing communities apart with its cruel immigration policies,” Representative Lucille Roybal-Allard, a California Democrat and conference committee member, tweeted Sunday. “A cap on ICE detention beds will force the Trump admin to prioritize deportation for criminals and people posing real security threats, not law-abiding immigrants contributing to our country.”

Appearing on Fox News Sunday, Shelby said talks are “stalled” over the Democrats’ request, which Republicans have been unwilling to endorse.

“We’re working and we’re hoping we can get there, but we’ve got to get fluid again, we’ve got to start movement,” Shelby said.

Acting White House Chief of Staff Mick Mulvaney went on “Meet the Press” Sunday and said he “absolutely cannot” rule out another shutdown. Mulvaney said whether the president would sign the eventual package put forward by the conference committee would largely depend on how much money it includes for a wall.

Also appearing on Fox News Sunday, Senator Jon Tester, a Montana Democrat who is on the committee, said discussions over money for a wall or other structure along the southern border are ongoing.

“We are not to a point where we can announce a deal,” Tester said. “Negotiations are still going on. There are good people on this committee, so I have confidence that hopefully we will get something done very soon.”

Negotiators face a tight deadline to get a bill on the floor. House leadership has notified members that it is possible the chamber will take votes over the weekend if it is unable to pass the funding package by Friday.

Members of the committee are slated to meet on Monday to continue negotiations.