Here's why the government could shut down
How can you have a government shutdown when one party controls both Congress and the White House? If it happens, it will be because Republicans couldn't keep key factions of their party happy or curry favor with Democrats over immigration — not to mention tweets undermining the negotiations from President Trump himself.
The bottom line: In order to pass the continuing resolution (CR) that will fund the government past its Friday deadline, GOP leadership must win over two key contingents: Senate Democrats and members of the House Freedom Caucus. Right now, it's hard to see the path that would allow them to satisfy both.