I agree with E.J. Dionne that whatever one thinks of the question of denying communion to pro-choice Catholic politicians, denying communion to Catholic citizens who express support for pro-choice Catholics politicians is a very, very bad idea. There is a crucial distinction between voting for a deeply-immoral measure as a legislator, and voting for a legislator who happens to support deeply-immoral measures; the former is never justifiable, but the latter course of action may well be, depending on the set of issues at stake in the election. Here I would cite these remarks from Denver Archbishop Charles Chaput, which Andrew called "metaphorically weird," but which (naturally enough) I found nicely phrased and essentially correct:
So can a Catholic in good conscience vote for a pro-choice candidate? The answer is: I can’t, and I won’t. But I do know some serious Catholics— people whom I admire—who may. I think their reasoning is mistaken, but at least they sincerely struggle with the abortion issue, and it causes them real pain. And most important: They don’t keep quiet about it; they don’t give up; they keep lobbying their party and their representatives to change their pro-abortion views and protect the unborn. Catholics can vote for pro-choice candidates if they vote for them despite—not because of—their pro-choice views.
But [Catholics who support pro-choice candidates] also need a compelling proportionate reason to justify it. What is a “proportionate” reason when it comes to the abortion issue? It’s the kind of reason we will be able to explain, with a clean heart, to the victims of abortion when we meet them face to face in the next life—which we most certainly will. If we’re confident that these victims will accept our motives as something more than an alibi, then we can proceed.
What this implies, I think, is that a pro-lifer can in good conscience support a pro-choice candidate only for reasons that go directly to issues of life and death on a scale proportionate to the scale of the abortion industry as licensed by Roe v. Wade. I think that this calculus points toward a distinction between foreign and domestic policy debates, with the former, as a general rule, offering much more plausible grounds for casting a vote for a pro-choice politician than the latter. In debates over foreign affairs, voters are asked to pass judgment on policies that often lead directly to the death of innocents on a large scale; the same simply cannot be said of most domestic controversies, at least in the contemporary United States, which is why the abortion issue weighs so heavily (and rightfully so) on the consciences of Catholic voters. Obviously many liberals would claim otherwise - i.e. a vote against S-CHIP is a vote to let thousands of children die! - but I think those arguments become very strained very quickly, especially when they're expanded to include putatively "life and death" issues like education, say, or the health of the labor movement.
In practice, this means I'm much more sympathetic to a pro-life Catholic who's supporting Barack Obama in order to vote against the life-and-death consequences of American interventionism, in Iraq and elsewhere - even though I'm skeptical about the merits of that particular calculus - than I am to a pro-life Catholic who's voting for Obama because he thinks the distribution of the American tax burden conflicts with Catholic social thought. Or again, even as I disagreed with their assessment, I would have been much more sympathetic to a pro-life Mondale voter who took the view that Ronald Reagan's foreign policy was raising the risk of thermonuclear war above and beyond any reasonable or acceptable level than to a pro-life Mondalenik who thought Reagan wasn't doing enough to maintain the "preferential option for the poor" that Catholic social teaching calls for.
Note, too, that this is a separate question from the issue of whether casting a pro-life vote actually has any chance to produce pro-life policies, or whether the dream of overturning Roe is essentially hopeless, and the alliance between pro-lifers and the GOP essentially fruitless. I can certainly imagine a circumstance in which the impossibility of the pro-life cause makes it plausible to vote for a pro-choice candidate with a clear conscience even when grave matters aren't at stake, but as I've argued before, I think we're some distance from that state of affairs.