Democrats have complained that this would be hypocritical since when Antonin Scalia died before the 2016 election and Barack Obama sought to appoint Merrick Garland, the Republican Senate refused to move on his nomination, effectively killing it. Even a few Republicans think it would be wrong to appoint a justice to the Supreme Court this close to an election.
Dan Mclaughlin at National Review vigorously disagrees. He opens his piece with these words:
Given the vital importance of the Court to rank-and-file Republican voters and grassroots activists, particularly in the five-decade-long quest to overturn Roe v. Wade, it would be political suicide for Republicans to refrain from filling a vacancy unless some law or important traditional norm was against them. There is no such law and no such norm; those are all on their side.President Trump is not only on solid legal ground, but also has precedent on his side in sending up a nominee as long as he's in office, even if he's a lame-duck after November:
Choosing not to fill a vacancy would be a historically unprecedented act of unilateral disarmament. It has never happened once in all of American history. There is no chance that the Democrats, in the same position, would ever reciprocate, as their own history illustrates.
...throughout American history, when their party controls the Senate, presidents get to fill Supreme Court vacancies at any time — even in a presidential election year, even in a lame-duck session after the election, even after defeat.
Historically, when the opposite party controls the Senate, the Senate gets to block Supreme Court nominees sent up in a presidential election year, and hold the seat open for the winner. Both of those precedents are settled by experience as old as the republic. Republicans should not create a brand-new precedent to deviate from them.
Twenty-nine times in American history there has been an open Supreme Court vacancy in a presidential election year, or in a lame-duck session before the next presidential inauguration. The president made a nomination in all twenty-nine cases. George Washington did it three times. John Adams did it. Thomas Jefferson did it. Abraham Lincoln did it. Ulysses S. Grant did it. Franklin D. Roosevelt did it. Dwight Eisenhower did it. Barack Obama, of course, did it.But should the Republican Senate use their legal power to seat a nominee the president sends them at this stage of Trump's presidency? Democrats argue that the Republican Senate refused to seat Merrick Garland before the 2016 election, and that it would be hypocritical of them to now seat a Trump nominee, but McLaughlan points out that the two situations are very different. In the case of Garland the presidency and the Senate were split with the former belonging to the Democrats and the latter to the Republicans. At present both belong to the Republicans.
Twenty-two of the 44 men to hold the office faced this situation, and all twenty-two made the decision to send up a nomination, whether or not they had the votes in the Senate.
So, today, Donald Trump has the raw power to make a Supreme Court nomination all the way to the end of his term. Senate Republicans have the raw power to confirm one at least until a new Senate is seated on January 3, and — so long as there are at least fifty Republican Senators on that date — until Trump leaves office.
Nineteen times between 1796 and 1968, presidents have sought to fill a Supreme Court vacancy in a presidential election year while their party controlled the Senate. Ten of those nominations came before the election; nine of the ten were successful, the only failure being the bipartisan filibuster of the ethically challenged Abe Fortas as Chief Justice in 1968.So, if a vacancy should occur, the Republicans have the right, both legally and in terms of precedent, to confirm Mr. Trump's nomination. Indeed, were the roles reversed the Democrats would certainly confirm a nominee sent them by a president of their own party regardless of when the vacancy occurred. For the Republicans to decline to do likewise would be a case of gross political malpractice that would alienate their voters for a generation.
McLaughlin has much more detail in his article, and interested readers are urged to check it out.