The Democrat Party most be worried about some Democrat Senators defecting to the Republican party. Got to create a bunch of new States.
I don't think they're worried about Senators defecting than they are about padding their majority. Admit Puerto Rico and D.C. and Schumer's majority becomes 4 seats rather than a tie and Pelosi picks up a couple of seats in the House.
It’s not at all clear that Puerto Rico would be a Democrat state, which is why both parties have steered clear of statehood forever. The Republican party in Puerto Rico has long advocated for statehood. The whole thing is a mess. Puerto Rico is beautiful, well at least before the hurricane, but has incredibly corrupt politics. But with Congress basically running the protectorate what do you expect?