A few Catholics have tried to argue that voting for John Kerry is better for the pro-life cause than voting for George Bush. They usually argue this in primarily two ways: first, Kerry's economic policies will reduce the need for and hence the number of abortions. Second, Kerry's position on the death penalty and the war in Iraq are more pro-life than Bush's positions on those issues.
There are a number of problems with these arguments, but rather than rehash them myself, I'll simply point you to this essay by Profs. Robert George and Gerard Bradley. It's a terrific read.