(CNN) 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.

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