Stocks jumped on Friday after President Trump "caved" to the Democrats and agreed to reopen the government via a three-week stopgap bill to facilitate negotiations over a border security package. But investors who interpreted this as a sign that Trump was giving up on his ambitions to build the wall - or that the president had decided prolonged government shutdowns were too politically damaging to risk a second one - might soon realize that this is simply not the case.

To wit, stock futures pointed to a lower open on Monday after President Trump warned on Sunday during an interview with the Wall Street Journal that he was personally pessimistic about the possibility that a bipartisan group of lawmakers will strike a deal that he would find acceptable. And if no deal is forthcoming, Trump vowed to build the wall anyway.

Though Trump said the group of 17 negotiators included some "very good people", Trump said he believed the chances of Congress averting another lapse in government funding were "less than 50-50." Asked if he would accept less than $5.7 billion in the next round of negotiations, Mr. Trump said: “I doubt it...I have to do it right.”

Responding to Democrats' insistence that they support border security measures like fencing, levees, bollard barriers, more immigration judges, technology and agents, Trump said he would be skeptical of any deal that trades wall funding for other border security measures. Asked if he would agree to citizenship for the so-called "Dreamers", Trump again said "I doubt it."

"That's a separate subject to be taken up at a separate time," Trump said, though he offered three years of temporary protections for the Dreamers as part of a deal he had offered the Democrats.

But most importantly, during the interview, Trump said he wouldn't rule out another shutdown: "It's certainly an option," he said, echoing comments from his Acting Chief of Staff Mick Mulvaney, who made the rounds on the Sunday shows to push back against the perception that Trump had somehow "caved" by offering to reopen the government.

Whatever agreement is ultimately struck - assuming the two sides can strike a deal - it will ultimately hinge on the nature of the border barrier that Trump has demanded since the earliest days of his campaign.

The thorniest question is likely to be what kinds of physical barriers would be funded along the border. Mr. Trump has been insistent on a wall in order to deliver on his signature campaign promise, while Democrats have been equally resistant to one. "Have I not been clear on a wall?" House Speaker Nancy Pelosi (D., Calif.) said Friday. "I’ve been very clear on the wall." Asked about the form of barrier he would accept on the border with Mexico, Mr. Trump said: "I have to see what it is. As long as it can stop criminals, gangs, human trafficking and drugs, I’m open to anything. But the only thing that will work is a very strong form of physical barrier." But Democrats and Republicans often use different rhetoric when describing the same provisions, suggesting they may be able to agree on some kind of funding that Republicans would consider a wall but Democrats would say constitutes border security.

If Trump goes the national emergency route, something that would inevitably be met with a legal challenge from Democrats (though Trump's record installations of federal judges and his recent appointments of two new SCOTUS justices would give him a slight edge), Trump could divert funding from military-funded projects like disaster relief.

On Sunday, Trump warned in a tweet that anybody who doubts his intentions to build the border wall will soon be sadly mistaken.