Should Catholics Vote for McCain?

That depends on whether or not Catholics believe that the Republican party can stop abortion. Now, Catholics must not vote for Obama, who is a pro-abortion politician. That much is obvious to the rank and file Catholic, although somehow there are a good deal of Catholics who feel like ending the war in Iraq is just as important as ending abortion.
To those types, let me point out that Barrack Obama is pro-war in Afghanistan, and has said "all options are on the table" regarding Iran. Obama is not anti-war. He has used an anti Iraq war stance to garner naive liberals into believing he is a peace president.
He is not.
So, back to John McCain III. Is this a man we want in office? Not really. Is the lesser of two evils a legitimate choice?
I'll leave that up to you ultimately. I don't believe the lesser of two evils is a good choice, and I'll explain. The lesser of two evils idea stems because:

1. There are only two real choices (a third party candidate will not miraculously win this election)
2. With Obama and a friendly Congress a ton of junk bills will get passed increasing government power and decreasing liberty in the United States

With regards to the first, I would like to point out that this has been said for decades. It's always the lesser of two evils we vote for in national politics. Every time. The 20th century has been a nonstop pragmatic decision. Each time we get a politician in office who doesn't deliver on their campaign promises, builds big government (yes, even Reagan did), and ultimately makes us worse off.
The less people that vote for the national candidates, the better the chance a third party candidate could make a showing. For example, approximately 60 million people voted for each of the two major party candidates. Getting another 60 million people behind a third party candidate would be an impossible endevour. But the less people who vote for the main two candidates, the more the gap is brought down to make a third party candidate more viable.

The second argument is more sound, although it is entirely possible that a rejection of John McCain and George Bush Big Government Republicans will bring about a grassroots swell of voters in the next Congressional election to bring in new blood. A hostile Congress with Obama in office might work like it did in 1994-and may bring about more sound policies.

In either scenario, abortion will not be stopped. As I have written before, it is praying the Rosary (something I've been remiss in lately) and volunteering in grassroots efforts (something I'm very remiss in) that will help end this horror. The Republican Party is not as powerful as the Catholic Church, and our loyalty is to the latter, not the former.

Our Lady of Guadalupe, pray for us