From the Wall Street Journal, by Gerald Seib

For months, Sen.  Ted Cruz has been hovering in the middle of the GOP presidential campaign pack, waiting for his moment—and honing his argument for why he has the strategy and appeal that can win for Republicans in 2016.

In a nutshell, the Cruz case is this: There is an army of silent evangelical voters out there, and I can mobilize them. The country has 90 million evangelical Christians, 54 million of whom stayed home on Election Day in 2012, he says. If I can get just 10 million of the no-shows to vote for me in 2016, we win.

Now that Mr. Cruz appears to be making his move—two recent polls show him rising to second place in Iowa, where evangelicals are strong—the question is: Does the math behind his argument hold up?

There is little doubt that a lack of enthusiasm among core voters hurt Republicans in 2012. Still, some in the party are dubious. Implicit in the Cruz case, they believe, is an assertion that mobilizing white evangelical voters is more important than the mission other Republicans put atop their agenda: doing better among the swelling ranks of minority voters, particularly Hispanics.

In a sense, then, the Cruz argument frames a strategic question for Republicans in 2016: Does victory lie in better mobilizing the existing GOP base, or in expanding that base to reflect changing demographics?

Mr. Cruz contends that Republicans lose when they nominate candidates—such as former Massachusetts Gov.  Mitt Romney—who fail to excite evangelicals at the core of today’s GOP. Some 30% of Americans are evangelical Christians, he argues, and the majority of them simply didn’t vote in 2012.

Certainly, low energy overall was a problem for Republicans when they failed to defeat a weakened President Barack  Obama in 2012. The under-enthusiasm was particularly noticeable among white voters, which seemed critical in at least one key state, Ohio. There, the white share of the vote declined to 79% from 83% in 2008, and Mr. Romney got fewer votes overall than  John McCain did four years earlier.

Mr. Cruz argues, in essence, that an energized based of evangelical voters, 76% of whom are white, would make up for such deficiencies, and he is hitting that math hard to mobilize evangelicals to his side. “Christians are staying home,” he told an audience at religiously conservative Bob Jones University in mid-November. “ Well mark my words, we will stay home no more.”

One shortcoming in the argument, though, is that almost half of those in the evangelical population that Mr. Cruz cites aren’t Republicans. An extensive survey of religion in America by the Pew Research Center showed that 28% of self-identified evangelical Christians are Democratic or lean Democratic, and 16% say they have no party leaning. Many evangelicals are African-Americans, a core Democratic audience unlikely to defect.

Meanwhile, many white evangelicals live in reliably red states that Republicans already are sure to win; a higher turnout there would simply run up a larger GOP margin. The broader question, then, is whether there are enough white evangelical conservatives to make a difference in the key states.

In a book titled “2016 and Beyond: How Republicans Can Elect a President in the New America,” GOP pollster  Whit Ayres—who works with Sen.  Marco Rubio’s campaign—writes that relying on finding “missing white voters” is “an excuse to avoid confronting the very real problems facing the Republican Party in a 21st century electorate.”

The number of white votes did drop by 4.2 million between the 2008 and 2012 elections, Mr. Ayres notes, and some of those were blue-collar Americans who didn’t warm to Mr. Romney.

But he also notes that if all those missing white voters had shown up and voted Republican, they still wouldn’t have saved Mr. Romney, who lost by 5 million votes.

The more important trend, he argues, is the steady decline of whites as a share of the electorate, and the steady rise of Hispanic voters. Whites as a share of the electorate fell to 72% in 2012 from 81% in 2000—and will fall further to 69% in 2016. Meanwhile, the share of minority voters will rise to 31% next year from 19% in 2000.

If the next Republican nominee doesn’t win more of the minority vote than the 17% Mr. Romney took, Mr. Ayres writes, he will have to win a hefty 65% of the white vote to prevail. That level hasn’t been hit since the  Ronald Reagan landslide of 1984.

Of course, the two arguments aren’t mutually exclusive: It’s possible to do better among white evangelicals and minorities. That’s a tall strategic order. Still, it may be that performing better among evangelicals is a necessary but not sufficient part of a GOP victory formula.