Canonical options for dealing with Catholic legislative support for FOCA
1. Canon 915. Make plain, by public announcement and/or private contact, that a legislator's support for FOCA qualifies as (probably formal, but certainly proximate material) cooperation with objective grave evil and that such conduct, in this case, would render one ineligible for reception of holy Communion under Canon 915.
This option requires little or no technical groundwork to be laid, carries immediate, visible, and salutary consequences (withholding of holy Communion from the publically unworthy and protecting the faithful from classical scandal), and, because it is a sacramental disciplinary norm and not a canonical penalty, it requires no formal process for imposition; finally, it leaves open the possibility of speedy reconciliation by a suitable expression of repentance.
More information: Raymond Burke, "The discipline regarding the denial of holy Communion to those obstinately persevering in manifest grave sin", Periodica 96 (2007) 3-58; Edward Peters, "Denial of the Eucharist to pro-abortion politicians", Homiletic & Pastoral Review (October 1990) 28-32, 48-49; and generally Brian O'Neel, "Just punishment", Catholic World Report (February 2002).
2. Canon 1369. Warn Catholic legislators that their support for FOCA appears to be using "a public show or speech [or] published writing . . . [to] gravely injure good morals", and that as such they would be liable to "a just penalty" under Canon 1369. The sanction need not be specified in advance, and contempt for any earlier sanctions can result in escalating penalties under 1983 CIC 1393.
This option requires little or no technical groundwork to be laid (no prior warning is necessary, but it might be pastorally prudent to offer same), and it carries visible and salutary consequences (ones flexible in nature, but which could eventually include excommunication). Because Canon 1369 is a penal norm, it would require a formal process (1983 CIC 1314, 1342) for imposition of the penalty. Canon 1369 can also be enforced by penal precept (1983 CIC 49, 1319, 1339).
More information: O'Neel, above, and generally Edward Peters, Penal Procedural Law in the 1983 Code of Canon Law, Canon Law Studies No. 537, (Catholic University of America 1991) 393 pp.
3. Canon 455. Enact at the episcopal conference level (though individual bishops are free to act here as well, per 1983 CIC 1315 et seq.) a "general decree" (1983 CIC 29, 455) making legislative support for FOCA a canonical offense and specifying a penalty or range of penalties.
This option requires that considerable groundwork be laid and, even if Roman authorization were forthcoming for conference action (I suspect it would be), there is probably not enough time to enact specific penal legislation before FOCA becomes an issue.
4. Be content to reiterate the Church's opposition to abortion and to allow individual Catholic legislators to decide for themselves whether support for FOCA is in accord with Church teaching and whether they feel comfortable in approaching holy Communion under Canon 916.
This option is essentially a decision to do nothing special in regard to FOCA. Those who consider this response pastorally sufficient are free to make a case for it.
Some additional points.
1. I do not think that a Catholic legislator's support for pro-abortion bills can result in automatic excommunication for abortion under Canon 1398 and, as I have argued elsewhere, I know of no canonist who holds otherwise. Notwithstanding Francis Cdl. George's and Deal Hudson's recent remarks about the possibility of excommunicating Catholic legislators over support for FOCA, I'm afraid that George and Hudson, in slightly different ways, have confused the criteria for assessing the morality of an act (which criteria they correctly outlined) with the criteria for assessing the legality of an act. Conscience and law are not in conflict here, but they follow, for very sound reasons, rather different ways of evaluating actions. In canonical matters, especially in penal ones (1983 CIC 18, 221) it is canonical interpretation that must be followed, and canonical penalties for abortion under Canon 1398 and 1329 do not reach beyond those directly and necessarily involved in the procuring of a specific abortion.
2. Options 1, 2, and 3 are not mutually exclusive.
3. A Catholic legislator's support for FOCA might suffice for incurring canonical consequences, be they sacramental or penal as above, regardless of whether FOCA actually becomes law. Moreover, some prominent Catholics not in legislative office might, by their public efforts on behalf of FOCA, leave themselves liable to canonical consequences for such conduct.
Update, 4 December 2008: See also Christian Brugger, "Denying Pro-Abortion Politicians Holy Communion: Some Considerations"