Both the New Republic and Politico have analyses of Harris v. Quinn the night before the opinion comes out.
The New Republic writes:

If the justices decide to rule for Harris and against SEIU, they could do so in a couple of ways. On the one hand, they could issue a relatively narrow opinion. Home health care workers are in somewhat unusual position, in part because the union bargains on their behalf by seeking greater reimbursements from Medicaid. The justices could decide that’s basically political activity, rather than more traditional collective bargaining activity, and invalidate the fees based on that basis. That would affect home health care workers and, perhaps, other workers whose incomes depend on direct payment from government programs. (Think childcare.) It might not apply to other public sector employees.

But the justices could also issue a broader ruling. They could decide it’s wrong to make any public employees pay the administrative fees for collective bargaining. That would essentially turn all public-sector workers into “right to work” occupations. There’s even a chancealbeit very remote, according to expertsthat the justices could extend their ruling to the private sector, arguing that exclusive representation by unions inherently limits constitutional rights to free speech and association.

Politico quotes Professor Ben Sachs extensively, explaining:

If the court sweeps away compulsory dues, many more unions “will need to convince employees to pay for representation that the employees would be entitled to receive for free,” said Benjamin Sachs, a Harvard law professor who studies labor and has blogged frequently about the case. “But unions have overcome difficult court decisions in the past,” Sachs said, “and the historical track record suggests that they will be able to adjust to a loss in Harris v. Quinn as well.”

Both are worth reading in full.