How does the Constitution limit the power to appoint a Supreme Court justice in the last year of a Presidential term? By putting one-third of the Senate, which must consent to the new appointment, up for re-election.
If the electorate in the key Senate races believes that no nomination should be made to replace Ruth Bader Ginsburg in 2020, the voters can exercise their power and even deny the President's party a Senate majority. And if the voters are unpersuaded that the issue is important, then the Senate majority will control the decision.
The Constitution makes this question intensely political, precisely because it is of such critical political importance to the country.