I've disagreed with him on immigration, global warming and federal protection of marriage. I've taken strong exception to his view that the federal government should fund embryonic stem-cell research. But disagreement on such issues is one of the reasons we have presidential primaries - so each party's voters can sort out the issues and personalities and choose the candidate who best reflects their collective view. Republicans have done that. Now the question for conservatives is whether McCain fits the Reagan Axiom that someone you agree with on 80 percent of the issues is your friend, not your enemy.

Of all the issues confronting the United States today, none is more important than our nation's security. Although these issues don't dominate our news as they once did, we cannot forget that without a safe and secure country, all other issues don't matter.

McCain is clearly the candidate with the capacity, judgment, experience and will to confront America's enemies. He's served our country honorably - heroically - in war. I served eight years with him on the Senate Armed Services Committee, and I can assure you he knows our military. Importantly, he also knows our enemies. He understands their capabilities and their aims. He will not sugarcoat the human or financial commitment and cost needed to defeat this enemy.

The most important social issue is life. Yes, I often wished McCain would have joined me on the Senate floor in debating Barbara Boxer on issues like the partial-birth-abortion ban. In the end, with the exception of embryonic stem-cell funding, he always voted for life and stood for the culture of life. In short, he's been a reliable vote on life issues, which are critical to conservatives.

Many conservatives have given McCain poor marks for his involvement in the Gang of 14. I was in leadership pushing hard for a showdown with the Democrats on using the "Constitutional Option" to end their filibuster of judicial nominations. The Gang of 14 broke the impasse, and it probably was for the best. I was the one counting votes on that issue, and I was much less certain of success than others. In the end, the Gang deal resulted in numerous confirmations of qualified conservative jurists.

On judges, McCain has repeatedly made clear that he will, as his Web site states, "only nominate judges who understand that their role is to faithfully apply the law as written, not impose their opinions through judicial fiat." Sounds good to me.

Yes, I disagreed with McCain's opposition to President Bush's tax cuts in 2001 and 2003. But I give him credit that he now thinks we must make those tax cuts permanent to boost our now-struggling economy.

And, yes, McCain has been a thorn in the side of many of us who supported important appropriations earmarks for our states. But he's always objected with principled consistency.

This is but one example of McCain's character - the kind of character I want in the person who answers 3 a.m. phone calls at the White House.

Those conservatives who still question whether they can support McCain should remember this: The next president will make more than 2,700 political appointments, those who really set policy, across the bureaucracy of our government. I, for one, will sleep better at 3 a.m. if Republicans are in the cabinet and in White House positions that make so many critical decisions. The idea of "Attorney General John Edwards" and "Energy Secretary Al Gore" should cause some sleepless nights for Republicans or conservatives - and those in a U.S. manufacturing sector now struggling to stay afloat.

Here's my final argument for John McCain. He's not Barack Obama or Hillary Clinton.

Both Democrats have made their case in chief on why they should be president, and we have every reason to be concerned.

Both want to cut and run from Iraq, give the radical jihadists a victory from the jaws of defeat, and leave the Iraqi people vulnerable to chaos. Both would put in place dangerous economic policies that would make Uncle Sam look like an Orwellian Big Brother. Both would nominate liberal activist judges who would pass undemocratic laws from the bench. Both support one-size-fits-all health-care policies that have been a disaster for patients and medical industries in Canada. Good-bye, American capitalism; hello, European-style socialism.

Many of my fellow Republicans have faulted me privately and publicly for being so outspoken about McCain, suggesting that I should have kept my mouth shut. First, I've never been very good at that. Second, I do not regret being up-front about such an important decision. Third, the primaries are a time when each party wrestles over what it's looking for in a presidential candidate. Now is the time to come together.

As for the Reagan Axiom, given his opponent, McCain is close enough to 80 percent for government work. That is why I am going to vote for my friend - John McCain.

Former U.S. Senator Rick Santorum is a Senior Fellow at the Ethics and Public Policy Center.