If you are pro-life, you can do no better than Rudy Giuliani as your candidate for president.
Consider that even if James Dobson himself were elected president, he could not sign legislation outlawing abortion - it would simply run aground on the Roe v. Wade precedent. All he could hope to do is appoint Supreme Court justices that adhere to a strict constructionist philosophy of constitutional interpretation, that might be able one day to overturn Roe. You cannot impose an anti-abortion litmus test on judicial nominees, because no nominee will ever state, in advance, that they will vote to overturn abortion precedents.
Rudy Giuliani has promised to do just that, appoint justices who believe in strict constructionism in the mold of Antonin Scalia and Clarence Thomas. Neither Mitt Romney nor Fred Thomspon could promise to do more for the pro-life view of things.
Consider what would happen if the evangelicals listened to James Dobson's horrible advice and considered voting for an independent candidate who, as I have just pointed out, can do no more for the pro-life movement than Giuliani has already promised to do. They will elect Hillary Clinton, and probably ensure that she appoints one or two SCOTUS justices within the first two years of her administration. Overturning Roe v. Wade will be put off for another generation at least, and if the sheer number of abortions is an ethical concern for you, ask yourself if you can sleep well knowing that your wasted vote ensured that millions more unborn human beings were murdered because you helped elect Hillary Clinton.
Not to mention that evangelicals will never be forgiven by the rest of the Republican Party for putting Hillary into the White House. You want all of that on your head? Then go ahead and pay heed to Dobson's destructive and extremely short-sighted advice. But don't say you weren't warned.