WASHINGTON — The final day of voting in this year’s midterm elections arrives Tuesday against a backdrop of uncertainty after a year in which President Donald Trump has tried to interfere in how states run their elections and sown doubts about the outcome should Republicans lose. Millions of Americans already have cast their ballots, with early voting turnout on pace to eclipse that of the last midterm elections two years ago. In contrast to the president’s continued rhetoric about election cheating, voting so far has unfolded with relatively few problems. Trump has spent months trying to use the power of the presidency to put his thumb on the scale of this year’s elections, which will determine whether his party keeps control of Congress. His Justice and Homeland Security departments have been involved in a massive hunt for voters who are not U.S. citizens, at one point engaging in a made-for-the-cameras predawn operation that led to more than two dozen arrests in California. The Department of Justice was planning its largest-ever effort to monitor Election Day polling places,