Peter Baker for the NYT:

President Trump insists he and his attorney general did nothing wrong by seeking damaging information about his domestic opponents from Ukraine, Australia, Italy and Britain or by publicly calling on China to investigate his most prominent Democratic challenger. But for every other White House in the modern era, Republican and Democratic, the idea of enlisting help from foreign powers for political advantage was seen as unwise and politically dangerous, if not unprincipled.
A survey of 10 former White House chiefs of staff under Presidents Ronald Reagan, Bush, Clinton, George W. Bush and Barack Obama found that none recalled any circumstance under which the White House had solicited or accepted political help from other countries, and all said they would have considered the very idea out of bounds.