Trump was not able to get (enough) border security funding, and so due to that disagreement, the government shut down. In particular, the Senate approved a bill without that border-funding, but the then Republican-led House decided to vote on a different bill.
All this happened before the Democrats took over the House with Nancy Pelosi being elected as House Speaker, whose first statements on her agenda were:
How many more times can we say no? Nothing for the wall.
It then stands to reason that getting the desired border security funding now is virtually impossible. It didn't happen before when Republicans were controlling both chambers, so it certainly isn't going to happen now. In fact, experts and commentators suggest that all Democrats need to do now is vote (in the House) on the bill that doesn't include border security funding which the Senate already passed.
And yet, we still see Republicans consistently making promises that the "Wall" will be built (actually, it's mainly just Trump who's talking about a "Wall", most others are smart enough to only talk about border security in general). It's all Trump can talk about on Twitter, and recently Lindsey Graham came out and said that
Why are Republicans going 'all-in', so to speak, on border security funding, when, most likely, as detailed above, they won't get it? It just seems political suicide, does it not?