I'll disclaim this by saying that I am a staunch conservative, and I am lucky enough to agree with almost everything in the Republican party platform. (Even though I despise or distrust a great many of its actual politicians.) However, I also sympathize very much with your position.
My mother was a Democrat for most of her life. Even though she was a Catholic and understood the voting principles in general, she couldn't bring herself to vote for a Republican, and pulled the lever for Carter, Mondale, and Dukakis in '80, '84, and '88. It wasn't until 1992 that that changed. Frustrated with the anti-life path the Democrats had taken, she attended a Democratic party caucus (we live in Minnesota, go caucuses!), caucused for the pro-life movement, got elected as a state delegate, and then got elected again as a national delegate, and went to the 1992 Democratic National Convention at Madison Square Garden in New York City, ready to cast her tiny protest vote for pro-life Democrat Gov. Bob Casey of Pennsylvania.
The Democratic Party wanted a unanimous nomination vote for Clinton. The pro-life delegation was, to put it simply, unacceptable. Their wardrobes were vandalized, they were harassed, they were expelled from their seats near the front row, and, in the end, they were physically beaten up by male heavies from the Minnesota delegation, then thrown out of the convention because death threats had been called in against them. (Walter Mondale sat in the row in front of the pro-lifers throughout the entire assault, and said nothing at all. He was a man without honor.)
So that was the end of my mom's sojourn as a Democrat. She conceded that (1) the Democrats were irredeemably pro-abortion, and (2) the Democrats really weren't the nice people she'd always imagined them to be. She started voting Republican in the next election, and gradually took on some Republican views, like tax cutting and Reaganism... but even today she has an entire lifetime of liberal political philosophy simmering just under the surface, and occasionally breaks out with some unexpected opinion that makes all her fashionable conservative friends look at her funny. (For instance, just tonight she was saying what a great idea it was for the local government to circumvent a referendum and impose a sales tax on a county in order to construct a giant downtown sports complex with government money. She's brilliant, really, but I just don't understand that point-of-view.) Even today, I am sympathetic to those who, like you, 2cats, are stuck with a choose between one party you think is basically good, but murderously mistaken on a few amazingly important issue, and another party that is actually composed of evil people.
I told that whole story just as a way of saying: I'm not in your boat, 2cats, but I get it.
My first recommendation is to reconsider Republicanism. It's a bit broken down at the moment, and I'm not all that sure that winning the midterms is going to help fix what's wrong with our party... but, at its best, Republicanism is a beautiful philosophy of governance that elegantly intertwines the limited government created by the U.S. Constitution with the principles of social justice mandated by Jesus Christ, to create a prosperous, happy, healthy, and free People. If you give the GOP a second, closer look, and you realize that it's actually a great party, then your problem is solved: you can vote GOP and like it.
My second recommendation, if the first fails, is to vote Republican anyway. There are a lot of really important issues today, from immigration to health care, poverty to the wars overseas, and the budget crisis. What Washington does about those issues matters enormously, and lives -- real lives -- hang in the balance. Nonetheless, against the vast ocean of evil that is legal abortion in this country, everything else, even if you wrapped it up in a great big multi-issue ball and threw it at Rush Limbaugh's head, all of it would add up to a raindrop in that ocean. In a race between a pro-life Democrat who was wrong about everything else and a pro-abortion Republican I completely agreed with, I would unhesitatingly vote for the Democrat. (This actually happened in the Minnesota governor's race in 1990.) Abortion, as my dad likes to say, is more than one major issue among several. It is not even enough to say that it is the biggest issue at present. Abortion is, both by its nature and by its scope, the dominating political issue. It is the alpha and the omega of political discourse in the twenty-first century. It must end. If that means you hold your nose and vote for somebody who's going to hurt the country or the state in a dozen other ways... I think you should do it.
However, you don't have to do it. The bishops have made some pretty clear statements that leave you and your conscience some leeway. Another option is to vote third-party, or to write in a candidate, or to run for office yourself. You can do that if you simply cannot bring yourself to vote for a Republican. I think you're wrong on several counts with that... but, you can take any of the alternative options and remain a Catholic in good conscience. What's more, if enough people feel the same way you do, eventually those third party candidates are going to become serious contenders. You may not feel like it at the time, but your protest vote does matter... especially in these shaky times, when confidence in the two parties is lower than it has been since Reconstruction.