It helps more Democrats than Republicans.
According to a Pew Research Center poll released this week, Democrats are now the party of college graduates, especially those with post-graduate work. Meanwhile, people with a high-school degree or less, by far the larger group, slightly lean toward Republicans.
Both preferences are the reverse of what they were in the 1990s.
https://www.washingtontimes.com/news/2018/mar/22/democrats-more-educated-republicans-pew-research-c/
Republicans have turned their backs on higher education. So, it's Democrats who are benefitting the most from student loan forgiveness.
This reason, of course, is on top of the usual hatred, cruelty, and social Darwinism of their party.