Trump is under no obligation to wait ’til the election. He should not.
There are two types of rules in Washington: laws that allocate power, and norms that reflect how power has traditionally, historically been used. Laws that allocate power are paramount, and particularly dangerous to violate, but there is no such law at issue here. A president can always make a nomination for a Supreme Court vacancy, no matter how late in his term or how many times he has been turned down; the only thing in his way is the Senate.
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. (This counts vacancies created by new seats on the Court, but not vacancies for which there was a nomination already pending when the year began, such as happened in 1835–36 and 1987–88.) 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. 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.
Threats are all the Democrats have – until the next Senate sits. And then, only if they take control.