As far as I can tell, the blogger Morning's Minion (MM), on the blog Vox Nova (for example, here and here), is one of many that have not grasped all of the teaching in (e.g.) the encyclical Veritatis Splendor. He says:

Take abortion and torture, two instrinsically evil acts. If one chooses torture instead of abortion on the grounds that it is the "lesser evil", then one is engaging in the kind of proportionalism condemned by John Paul in Veritatis Splendour.

Firstly, one can never choose to perform any evil, under any circumstances. So, one can never perform abortion, and one can never perform torture. Choosing one over the other still leaves an evil act being performed: whichever act is chosen, it is still evil, and no appeal to proportionalism is needed to see this.

When MM refers to "the lesser evil", this is the principle that a lesser evil can sometimes be tolerated, provided it occurs as the unintended and indirect consequence of directly choosing a greater good (or avoiding a greater evil). It may be that MM is only intending to cover that case. But what is that case?

A classic example: an out-of-control train is traveling down a track which will shortly fork into two directions. The driver can choose to go left, with the result that one innocent person will be killed. Or the driver can go to the right, with the result that ten innocent people will be killed. Moral theology allows that the driver can deliberately choose to go to the left.

This is sometimes called "choosing the lesser evil" (one person dies rather than ten), but this can be a very misleading terminology. It is much clearer to call it "choosing the greatest possible good". By going to the left, the driver is choosing to save ten innocent people — and saving people is a good thing to do. The driver's choice in going left is good. The choice does imply that one innocent person will die, but this is unintended, indirect, and the chosen good is proportionately larger than the foreseen evil.

It's in this sense that one can choose a lesser evil over a greater one — provided the lesser evil is unintended, indirect, and occurs through some kind of force (in the same way that the train was out-of-control).

MM goes on:

But if the act of voting itself is not intrinsically evil, and you do not share in the intent to either abort babies or torture people, then proportional considerations can come into play.

Before looking at what proportionate reasons there might be, it is well to consider that fact that the support for abortion can potentially include three intrinsic evils:

1. it encourages abortions to actually be carried out;
2. it fails to put into place the protection of the innocent from murder;
3. it proclaims that an evil is a good.

MM then says:

a legitimate prudential judgment could well be to support the pro-abortion candidate on the grounds that he or she would not affect any increase in abortion

While that would take into account reason (1) given above, it does nothing to avoid the evils of (2) and (3).