WASHINGTON (Reuters) - With time running down, congressional negotiators on U.S. border security funding have not settled the hot-button issues that confront them, but lawmakers expressed optimism on Monday that a deal could be struck before a Feb. 15 deadline.

FILE PHOTO: The Boquillas Crossing Port of Entry station at the re-opened port of entry into the town of Boquillas del Carmen, Mexico, from Big Bend National Park, Texas, U.S.. REUTERS/Jessica Lutz

“I think there’s a real effort by both Republicans and Democrats, both in the House and Senate to come together - more so than I’ve seen in a long time,” said Senator Patrick Leahy, one of 17 negotiators, and a senior Senate Appropriations Committee Democrat.

Republican Senator Richard Shelby, who chairs the appropriations panel and is also one of the negotiators, told reporters that border patrol experts would brief lawmakers soon.

“We are going to find out what they think ... what do we need on the border,” adding: “We’re trying to create a dynamic to bring us together.”

Such positive talk was still a ways from nailing down a deal. The toughest unresolved disputes include the type of new physical barriers along the U.S.-Mexico border, if any, as President Donald Trump demands $5.7 billion for a wall that most Democrats and many Republicans say would be wasteful and ineffective.

Trump has warned lawmakers they are “wasting their time” if they are not discussing “a wall or physical barrier.”

Shelby warned that if the lawmakers failed to agree, he thought Trump would declare a national emergency, using money already appropriated by Congress for other activities to build his wall.

While Democrats and several Republicans have warned Trump against doing that, Shelby said he would support such a move by Trump, although the senator preferred a legislative solution.

Republican Senator Roy Blunt, also a border security negotiator, said: “There are significant constitutional questions about the president’s ability to just decide that the Congress is wrong and he’s right.”

Other difficult questions facing lawmakers, according to congressional aides, include whether to increase or cut funds for immigrant detention beds and the numbers of immigration law-enforcement agents and immigration judges.

Democrats had been backing legislation providing up to $1.6 billion for additional fencing on some parts of the border, far below Trump’s request for a wall that he originally envisioned as a 2,000-mile (3,200-km) concrete barrier.

But new fencing money was not included in an initial proposal Democrats sketched out last week, which did call, however, for a $589 million increase in DHS’ budget.

Some liberal Democrats potentially complicated matters by pressing for cuts in homeland security spending.

Representative Pramila Jayapal, co-chair of the liberal Congressional Progressive Caucus, told reporters she agreed with sentiments expressed in a letter circulated by four members of that group, which seeks Department of Homeland Security funding cuts.

Dated Jan. 29, the letter said the Trump administration had “abused their authority and the fidelity of public resources,” with initiatives that included separating immigrant children from their families.