I have a question; say one of these candidates is elected in the primary, would you not vote for him and let someone like Hillary Clinton or John Edwards become president?
Yes.
1) Voting for a candidate who condones child killing profanes God, no matter how much more wicked the other candidate is. We should not do evil that good might come of it. We should not vote for a Nazi because he promises to kill fewer innocent people than the Stalinist he's running against.
2) The Republican Party has become increasingly liberal. If we vote for whoever they nominate, just so long as that candidate is a smidgeon less liberal than the Democrat he is running against, then we give them no incentive for nominating worthy candidates. They will continue to nominate wicked candidates, wooing "swing voters" in the middle while taking our votes for granted. The only way I can think of to pull the Republican party back to the right is to stop voting for their liberal pro-abortion candidates. Finding a right-wing third-party candidate to vote for is better than not voting at all, because then there will be a record of how many right-wingers were dissatisfied with the Republican candidate.
3) When a liberal Democrat is in office, the Republican politicians and the Christian leaders oppose their liberal agenda at every turn. But those same people will go along with the same liberal policies if they are put forth by a Republican because he's "our guy." We will call him "pro-life" no matter how much he undermines the personhood of the unborn, no matter how many "exceptions" he sites where he thinks abortion should be permissible, and he can even say that a state should get to decide whether abortion is legal. He will still be labelled "pro-life.
For example, George W. Bush has consistently stated, even before he was elected to his first term, that he thinks abortion should be legal in cases of rape and incest. When South Dakota recently had a referendum on its ballot to make all abortion illegal, Bush campaigned against it because it did not reflect his position. Yet Christians leaders identify Bush as "pro-life." James Dobson even calls Bush our most pro-life president ever.
Another example: Justice Scalia pledges that while he would overturn Roe v. Wade, he would also strike down its opposite: a law that would make abortion illegal in all states. He openly states that he thinks the people within a state should get to decide whether abortion is legal in that state. Yet Christians are deaf to this and they constantly label him as pro-life just because he disagrees with Roe v. Wade.
The same goes for Thomas, Roberts, and Alito. They are all pro-choice by state, and they are legal positivists who undermine the personhood of unborn babies. But since they were appointed by Republicans, they are labeled "pro-life." They give a ruling detailing several ways in which a baby can be slaughtered without violating the so-called partial-birth abortion ban (which never had the hope of saving a single child's life), and the Christian pro-life movement celebrate it as a victory.
We have been voting Republican for the past generation and as a result we have seven Republican-appointed Supreme Court justices, and zero pro-life justices. And in general, Christians have no idea because our leaders keep telling them otherwise.