(By Boris Heersink, a Ph.D. candidate at the Woodrow Wilson Department of Politics at the University of Virginia and a National Fellow at the Miller Center)  Donald Trump’s surprising and ongoing role as the de facto frontrunner for the Republican presidential nomination has raised considerable concern among Republican leaders. At this point, they may not yet be worried that Trump will actually become their nominee. After all, the history of presidential nominations is full of one-time frontrunners who failed to make it all the way.