A candidate's strengths can also be his weaknesses. Take the case of Rick Santorum.

One of his strengths is perseverance. For more than a year he made hundreds of appearances in Iowa, New Hampshire and South Carolina, with no visible result in the polls.

He persevered and ended up finishing first in the Iowa caucuses on January 3. Then, after poor showings in New Hampshire, South Carolina, Florida and Nevada, he finished first in Missouri, Minnesota and Colorado on February 7.

Now he's leading Mitt Romney in most polls nationally and in Romney's native state of Michigan.

Santorum's other strengths include spontaneity and authenticity. His speeches are unscripted and he answers, often at considerable length, every question at campaign events.

And those answers are sometimes not what any competent political consultant would recommend. Which is where Santorum's strength becomes a weakness.

"Contraceptive use is not a public policy question, and in bringing the subject up Santorum sounded like he is running for preacher or pastor." -- Michael Barone

Example: In an interview last October with the evangelical blog Caffeinated Thoughts, Santorum said, "One of the things that I will talk about that no president has talked about before is I think the dangers of contraception in this country." Contraception, he went on, is "not okay."

"Maybe people don't want us to talk about those things," he added. And he has said later that he doesn't seek a ban on contraceptives -- a good thing, since that was ruled unconstitutional by the Supreme Court in Griswold v. Connecticut 47 years ago.

But by bringing the subject up, he guaranteed that he would be peppered with questions about the issue by the likes of ABC's George Stephanopoulos and CBS's Bob Schieffer.

More recently Santorum opined that Barack Obama had a "phony theology." The context showed he was referring to Obama's environmental policies, and he later said he doesn't doubt the president's claim to be a Christian. But his ad libbed use of the word "theology" inevitably caused controversy.

No one can doubt that his opposition to contraception and his recent denunciations of prenatal testing and women in combat reflect his deep moral and religious beliefs.

But they also allow opponents to pigeonhole Santorum as a religious conservative despite his considerable record on and knowledge of economic and foreign policy issues.

It is political malpractice to give opponents such an opening in a year when voters are overwhelmingly focused on the economy and the Obama Democrats' vast expansion of the size and scope of government.

It's unfortunate also since Santorum sometimes make similar points in a less inflammatory manner. On the stump he often cites a Brookings Institution study that shows that virtually all of those who graduate from high school, get a job, and marry before having children escape from poverty.

It's a valid argument, one made some years ago by my American Enterprise Institute colleague Charles Murray and emphasized in his recently published book Coming Apart: The State of White America, 1960-2010.

As Murray shows, much of the income inequality that political liberals decry results from bad personal choices and behaviors rather than the operations of the market economy.

But it's unclear what presidents, much less presidential candidates, can do to influence these personal choices and behaviors, beyond setting a good example in their personal lives, as Obama, Santorum and Romney all do.

"I'm not running for preacher," Santorum said in his Caffeinated Thoughts interview. "I'm not running for pastor, but these are important public policy questions."

But contraceptive use is not a public policy question, and in bringing the subject up Santorum sounded like he is running for preacher or pastor.

Mitt Romney took a different approach when George Stephanopoulos raised the subject in the January 8 New Hampshire debate. "Contraception. It's working just fine," he said. "Leave it alone."

It's unlikely that John King will leave contraception alone in tonight's CNN debate in Arizona. Or that he will fail to ask about Santorum's "phony theology" charge.

Voters often say they value authenticity and spontaneity in candidates, and Santorum gives them plenty of that. And they admire perseverance in the face of adversity.

But they also want a certain amount of self-discipline in their officeholders, and particularly in their presidents, and they want them to focus on public policy issues they consider important.

At his best moments in the campaign -- in his Iowa caucus night speech, in the second South Carolina debate -- Rick Santorum has shown such discipline and focus. He needs to do that again.