Inside Catholic offers us the most unambiguous declaration possible:
As Catholics we are faced with a number of issues that are of concern and should be addressed, such as immigration reform, healthcare, the economy and its solvency, care and concern for the poor, and the war on terror. As Catholics we must be concerned about these issues and work to see that just solutions are brought about. There are many possible solutions to these issues and there can be reasonable debate among Catholics on how to best approach and solve them. These are matters of "prudential judgment." But let us be clear: issues of prudential judgment are not morally equivalent to issues involving intrinsic evils. No matter how right a given candidate is on any of these issues, it does not outweigh a candidate's unacceptable position in favor of an intrinsic evil such as abortion or the protection of "abortion rights."In other words, were the most incompetent, corporation-loving, money-hungry, warmongering bastard (i.e. a Republican) in the history of government to step forward and were he to state on record that he supports the legal protection of innocent human life from the moment of conception, he would still be more deserving of our vote than Obama.
To get this one thing wrong renders a candidate null and void.
As Forming Consciences for Faithful Citizenship states:"The direct and intentional destruction of innocent human life from the moment of conception until natural death is always wrong and is not just one issue among many. It must always be opposed."