In my book, it certainly is grave matter.
Sometimes, there is no clear choice between good and evil. Sometimes one may need to try to discern the lesser of two evils.
To vote for a pro choice will somehow involve one in what results from that choice and in this case abortions; however, there might be other issues in voting for the politician who is pro life - issues that do not protect human life rather mortally endanger it; other issues the pro life politician promotes that endanger seriously the common good.
I like the following post by @Tis_Bearself:
"We’ve had a couple dozen threads on this already, but I’ll say it again:
Voting for Biden is NOT a mortal sin unless a voter were to choose Biden, with full knowledge and full consent of the will, for the main purpose of opposing the Church teaching on abortion OR OTHERWISE UNDERMINE CHURCH TEACHING (formatting are mine).
Those who claim voting for Biden is always a mortal sin are going against the teaching of the Church. There are many legitimate and moral reasons for a voter to choose to vote for Biden.
Not to mention that we do not have full insight into the knowledge and consent of others, so we’re not in a position to tell someone else they committed a “mortal sin” unless perhaps we are the priest hearing their confession and gathering this information from them, in which case we wouldn’t be allowed to talk about their mortal sins outside the confessional anyway."
Reading matter on The Common Good:
https://www.catholicsocialteaching.org.uk/principles/glossary/ (scroll to Common Good)