It’s the only thing that matters, and yet, Republicans – even lead negotiators – have little guarantee that President Donald Trump will sign any border deal they can pull together. The signs are promising that a 17-person conference committee is making progress. After a private meeting with border officials Wednesday, Republican and Democratic lawmakers alike emerged with that faintest tinge of optimism that had all but evaporated from the Capitol last month during a 35-day shutdown. Aides and members point to progress: Democrats and Republicans are trading proposals back and forth, negotiating in good faith and trying to find a commonsense middle ground that includes a three-pronged approach to border security: a mix of barriers, personnel and technology that both sides can agree on. “What we are hearing right now is that they are talking. It’s not like some of these negotiations where they all come together 24 hours before a deadline and say ‘we’ve reached an impasse’ when in fact they haven’t even had any meaningful discussions,” Republican Sen. Thom Tillis of North Carolina told CNN. “I know that they are consulting with (the White House), but I don’t know what level of depth they’ve gotten into.” After more than two years in office, Trump’s Republican colleagues in the House and Senate have resigned themselves to the fact that the President can be impulsive, easily swayed by the counsel of a few rabble-rousing members of the House Freedom Caucus or convinced by an unfavorable review on cable news from an immigration hardliner. It’s why even as negotiations progress and appropriators – known less for ideology than for their pragmatism on Capitol Hill – remain upbeat about the potential for a deal, lawmakers are always accounting for the fact Trump is a wild card. “I believe that if we get to an agreement, there is a good chance he will sign it. Now, will he sign it? I don’t know,” Senate Appropriations Chairman Richard Shelby, an Alabama Republican, told CNN. Shelby told CNN he would meet with Trump – whom he referred to as “the big man” – at the White House Thursday morning to discuss the conference committee. Following the meeting, Shelby was visibly pleased and upbeat when he returned to the Capitol. He told CNN that he and the President did not discuss an overall figure for an emerging agreement but rather the substantive makeup of what they were discussing – new barriers, new technology and new personnel. He indicated Trump was aware that three-pronged approach is what was being formulated by the conference committee and that he thought Trump was ready to sign an agreement like that and keep the government open. “I am pleased,” Shelby said. “We had a positive meeting. The President was urging me to try to conclude these negotiations. This is the most positive meeting I’ve had in a long time. So, we’re not there yet, we’re seriously negotiating.” One Republican senator who asked to speak on the condition of anonymity to discuss internal GOP thinking, said there’s really one big question that worries everyone. “I think the question hanging over everybody’s head is whatever deal they do – and I think they do a deal – will it be acceptable to the President?” the senator told CNN. South Dakota’s John Thune, who as the majority whip is the No. 2 Republican in the Senate, said he’d just prefer not to entertain that hypothetical. “I just haven’t thought about that scenario,” said Thune. The fear comes after the President blindsided Republicans in December by refusing to sign a stop-gap funding measure to fund the government even after it had sailed through the Senate. In the weeks that followed, Majority Leader Mitch McConnell, a Kentucky Republican refused to take up any legislation the President hadn’t endorsed. In another test of trust last year, the White House torpedoed a bipartisan immigration deal that some of the President’s closest allies on Capitol Hill – including South Carolina Republican Sen. Lindsey Graham – had worked on. While nothing can stop Trump from tweeting, this time congressional negotiators have largely tried to silo themselves off from too much influence from leaders. That’s not to say that there aren’t sticking points: there are still details to be worked on not only how much money will be devoted to border barriers, but also how many detention beds to fund at the border and where to add staffing. Congressional negotiators had hoped to come to an agreement by Friday to allow enough time for the legislative process before the February 15 deadline, but that deadline could slip. Still, there’s no denying there’s ultimately just one opinion that matters, and that is the President’s. House Majority Leader Steny Hoyer told reporters Wednesday he hopes the President would sign any agreement reached by the conference committee. “Am I sure of that? No. Why? Because he’s changed his mind,” Hoyer said, pointing to the past examples of the President backing away from bipartisan agreements. Lawmakers hope that the bruising shutdown battle would be enough to encourage Trump to sign any proposal they send him. Reminded that hasn’t always deterred the President, one Republican member countered “that was then” before 800,000 federal workers were furloughed or working without pay, and the President was forced to reopen the government a month later without any money for his border wall. Trump is still holding onto the option of declaring a national emergency, a step that Republican leaders have warned him could divide their party and force a public rebuke of the President on the Senate floor. Sen. Mitt Romney, a freshman Republican from Utah, said he hoped his party could “reach a legislative solution” rather than depend on Trump to take executive action on a border wall. “It’s been conveyed to him pretty clearly,” one Republican said requesting anonymity to discuss the ongoing conference discussions. “The message has been pretty clear that there are lots of issues with Republican senators on this.” This story has been updated with additional developments.