Abortion is not the only moral issue to consider when you decide how to vote. You may also be concerned about education, healthcare, the distribution of wealth, the environment, foreign policy, investment in infrastructure, and so on. You may consider that on balance you are doing the better thing by voting for a pro-choice candidate whose views better align with yours on every other issue. Politics is never going to be simple.
I live in Great Britain (the situation is somewhat different in Northern Ireland), where all of the major parties take a more or less liberal stand on abortion.