Many reasonable conservatives will grant that Trump is not one of them. They will grant that he has no interest in competitive markets, in restraining spending, in protecting the family, in leaving issues to the states, or defending religious liberty. They may even admit that he is ill-informed, incompetent, and a serious threat to the system of alliances that has kept the western world at peace for the past 70 years. And yet, they argue, the power of SCOTUS demands that they fall in line."You have no choice. You've gotta go for Trump. Supreme. Court. Justices." Trump's argument, basically, is: You losers are stuck with me.— Peter Suderman (@petersuderman) July 22, 2016
Anyone expecting Donald Trump to be the Republican President to achieve the overturn of Roe v Wade, after four of his predecessors (committed to the issue, where he is not) have tried and failed, must be counted as somewhat deluded. As must any Republican who expects him to nominate Supreme Court justices who will institute tight limits on the power of the presidency and federal government. Even if the Republican Party retains the Senate in November and he dutifully defers to Mitch McConnell's list of suggested nominees (don't laugh!), midterm elections will eventually give the Senate back to the Democrats. If Trump doesn't cut a deal with Chuck Schumer to seat a pro-Planned Parenthood liberal before, he certainly will then.
And, if Trump is elected President, that means he will be the Republican nominee in 2020. As a result, in the best case scenario it will be 2024 (if ever) before conservatives get a chance to put a solid choice of their own on the court. A desperate haste to salvage a terrible deal this year will prevent conservatives from striking a good one from 2020.
With Hillary Clinton as President, the Democrats will very likely lose the majority of the Senate in 2018 if they win it back at all. A Senate GOP dominated by conservatives, united in opposition to Hillary rather than subservient to Donald, is one that can be relied upon to take a firm stance on SCOTUS appointments (as it has done so this year).
Under divided government, Presidents hoping to change the composition of the Court must yield to the preferences of the Senate majority party if they wish a new justice to be seated. This is how liberals achieved Souter, Stevens, and Kennedy -- nominally appointed by Republican presidents. No additional justice need be appointed to the Supeme Court unless the majority of the Senate feels that the Court is improved as a result. The Senate can hold firm to insist that a compromise pick be nominated.