What’s different for them today or when GOP was insisting on Obamacare defunding? The issues. Democrats do not, as far as I can tell, have any objection to the continuing resolution (which needs 60 votes in the Senate) or CHIP funding. They are shutting down the government over DACA, which was a temporary solution enacted by fiat by Obama (when unilateral executive governance was celebrated). Now, Democrats can get their DACA deal in a few weeks if they agree to compromise with Trump and give him his wall — a wall that Senate Minority Leader Chuck Schumer, among others, has voted to fund in the past. You may like the idea of a more secure border wall or you may not, but Democrats lost a presidential election largely predicated on this issue, not to mention Congress. So that’s the price of a deal right now. If they don’t want to negotiate, they should win the presidency, the House and a supermajority in the Senate. Or, they can instigate a shut down.
No matter what position they find themselves in, however, it seems that Democrats treat all their positions as moral imperatives, and any pushback as apostasy rather than disagreement. They think of policy aims as you might think of natural rights.