As Jonathan Adler notes, the Supreme Court today decided to hear Trump v Sierra Club, one of several cases that have questioned President Trump's diversion of various funds to build his border wall. The Ninth Circle court ruling in this particular case addresses Trump's efforts to use approximately $ 2.5 billion in military construction funds to build portions of the wall under Section 8005 of the Department of Defense Appropriations Act of 2019. It should not be confused with a similar case (which was also ruled by the Ninth Circle and in which most of the parties were involved) that ruled against Trump's efforts to divert military construction funds using 10 USC Section 2808, one of several triggered powers through Trump's declaration of a "national emergency" on the southern border. Unlike Section 2808, the current case does not address the crucial issue of the extent of the President's "emergency" powers.
Typically, a Supreme Court decision to hear a case is not good news for the party that prevailed in the lower courts (in this case, the plaintiffs who oppose the border wall). It is certainly possible that the Conservative majority in the Supreme Court decided to take the case because they wanted to override the ninth circuit on the matter – i.e. H. conclude that Trump was empowered to use Section 8005 to divert the funds. However, other scenarios are also possible.
Both in this case and in an earlier preliminary ruling by the Ninth Circle on the same subject, the dissidents of the lower courts focused primarily on the procedural argument that the plaintiffs lacked an appropriate "cause of action" to contest the diversion of funds in the case. Indeed, this is one of the questions that the Supreme Court has examined. If they terminate the ninth circuit for procedural reasons, the possibility remains that other parties can continue to challenge the diversion of funds and take a decision on the matter. For example, the US Court of Appeals for the DC Circuit – in a ruling by prominent Conservative Judge David Sentelle – recently ruled that the democratically controlled House of Representatives is entitled to pursue a constitutional separation of powers claim.
A decision on the matter in favor of the government would be an important victory for Trump. But it might still leave open the possibility that he can transfer money using section 8005 but not section 2808, thereby denying him access to some of the pots of money he is trying to use.
After all, it is possible that the Court of Justice took up the case because a majority of the judges want to rule the matter against the administration. In this scenario, the judges may want to send a strong signal that presidents must not be quick and easy to play with purchasing power that properly belongs to Congress.
Many observers seem to assume that this issue will inevitably divide judges on an ideological basis. If so, the plaintiffs will lose. But not all judgments of the lower courts have split in this way. The dissidents in several of the Ninth Circuit at the Wall Bypass were both Conservative Republican officials, in this case Judge Daniel Collins and the Section 2808 case, and Judge N. Randy Smith in the injunction on the current case. On the other hand, Judge Richard Clifton, a George W. Bush-appointed judge, had a majority in the latter decision. And, as mentioned earlier, prominent Conservative Judge David Sentelle authored the DC Circuit's most recent ruling against the administration in the case filed by the House of Representatives. Assuming the three liberal judges are likely to vote with the plaintiffs, it will only take the latter two conservative "defectors" to prevail.
It is also possible that the entire case could be discussed before the Supreme Court has a chance to rule on it. If Joe Biden wins the presidential election, he will likely simply end all efforts to divert funds as he promised.
Otherwise the court has to make a decision. If they get the merits, it could set an important precedent for the separation of powers and the authority of Congress over federal spending. The stakes in this case go far beyond the specific problem of wall construction. If Trump has near-limitless powers to divert various types of construction funds to build the Wall, future presidents can use the same power to fund all sorts of other projects.
I have discussed in more detail here, here, and here the broader issues that the wall disputes are about. Some of them are not directly involved in the case of Section 8005. For example, Section 8005 does not address the question of whether Trump can use the power of a significant domain to seize property for the border wall and whether his declaration of a "national emergency" is legal.