What is the average voter turnout among Democrats in U.S. midterm elections? How does it compare to Republican turnout?
FiveThirtyEight did an article on this. They found that there was a strong midterm advantage for Republicans when a Democrat was president. However, when a Republican is president, the advantage mostly disappears.
Their turnout measurement was the difference between the two party registration margin and the two party ratio in the voting. They found that when a Democrat was president Republicans turned out at rates from +3% to +6% relative to their registration. They found out that when a Republican was president, the turnout advantage ranged from none to +3%.
Presidential and Congressional popularity mattered here. The worst result when a Democrat was president was +3% in 1998 when voters were annoyed with Republican attempts to impeach Bill Clinton. The best result when a Republican was president was +3% in 1990 when George H. W. Bush was basking in the high approval that came from the success of the Gulf War.
The median value when a Republican is president is an advantage of +1% for the Republicans.
It's worth noting that this method of measuring turnout controls for registration changes. However, it is just as useful to the party to get a registration advantage as a turnout advantage. Registration tends to favor the party in the White House (of course, Donald Trump may be different). So depending on why you want to know, this may or may not provide the information you seek.
It is also unclear how popular Trump will be in November (I'm writing this in May of 2018). If he just signed a North Korean peace deal, he may be very popular. If there was just a stock market crash, he might be quite unpopular. His popularity is currently low but might either improve or get worse in the next five and a half months. Either way would provide a new data point.