Great question. In metonymy, the thing substituted isn’t really a part of what you’re talking about (“the suits in the corporate office”, or “I lost money at the track”), you have to mentally substitute an evocative thing, so Ds think metonymy since the wall isn’t part of actual enhanced border security.
And when you can’t even agree on the basics, you have little basis for negotiation.
Democrats Try to Box In Trump With Plan to End Government Shutdown Without Wall Funding
By splitting off the homeland security bill, Democrats are opening the door to a month of negotiations. But they are also essentially daring Mr. Trump and Senator Mitch McConnell, Republican of Kentucky and the majority leader, to keep a big chunk of the government shuttered over the president’s demand for the wall. Senator Chuck Schumer of New York, the Democratic leader, and Representative Nancy Pelosi of California, who is expected to be sworn in as speaker on Thursday, challenged Republicans in a joint statement on Monday.