Over the past month, former Alaska Gov. Sarah Palin (R) has quietly altered her case for the presidency in a savvy shift of rhetoric.

The lofty goal? Turning the main reason a Republican might oppose her into the primary reason for voting for her. If she can pull it off, the nomination might be hers, but she is up against a tough set of barriers.

Every candidate must have a compelling reason for running, and until one month ago, Palin’s was simple.

In October, she told “Entertainment Tonight” that she would only run if the field were missing a candidate who had “common sense” and “pro-Constitution passion.”

It there were such a candidate, Palin would opt out of a race and be “their biggest supporter and biggest help-mate.”

That answer boxed Palin in considerably, because the field will likely include at least a few candidates who fit those criteria. For example, Palin gave a ringing endorsement to Texas Gov. Rick Perry (R), a possible 2012 candidate, in his reelection bid earlier this year.

“He does what is right regardless of whether it is popular. He walks the walk of a true conservative. And he sticks to his guns — and you know how I feel about guns!” she said of him at the time.

And there are other names closely associated with the Tea Party movement, of which Palin is a prominent member, who might run: Rep. Mike Pence (R) of Indiana, former Pennsylvania Sen. Rick Santorum (R) and former Speaker of the House Newt Gingrich (R-Ga.).

Palin’s criteria for running were so rigid that former New York City Mayor Rudy Giuliani publicly scoffed at them while appearing on ABC’s morning chat show “The View.”

When co-host Barbara Walters told him of Palin’s terms and conditions, Giuliani said: “Well, first of all she’s not going to have that situation — that nobody else will [run]. I ain’t never heard of that.”

The question is why Palin would have applied such strict criteria to run, and the answer might involve her political image. Palin has consistently positioned herself as a reluctant candidate, someone who would only run if called to by unusual circumstances.

Earlier this year, she told Glenn Beck: “What we need to seek in a candidate [is] someone — I’ll repeat this — almost reluctant to serve. … I would be perfectly happy to go back to Wasilla, Alaska, with my five children and my grandson and raise a happy, healthy family.”

But, she added, “If I believe that in some capacity I can help this great nation, I’m going to be willing to sacrifice … in order to serve.”

So, until recently, Palin took a somewhat passive approach to running, in effect saying it would take the extraordinary circumstance of a field devoid of conservative candidates to pry her from a serene post-political life.

But last month, Palin tweaked her criteria — unannounced and, largely, unnoticed.

During an interview with Fox News, Palin repeated that she wanted to see if there were other conservative candidates who would run before making her decision, but slipped in this tiny proviso: Those candidates had to be “electable.”

“I’m certainly going to take a good lay-of-the-land look and see if there are others out there who are electable,” she said.

And that doesn’t seem to be a one-time shift in rhetoric. During an interview with Barbara Walters that aired on ABC last week, she was even more explicit in claiming that electability was the key determinant.

“I would run if I believe that other candidates willing to put themselves forward in the name of public service — if they don’t have a shot at winning, I would offer myself up.”

So under Palin’s new criteria, it doesn’t matter whether there’s a constitutional conservative already in the race. It only matters if he or she is electable.

Notice the second, subtle thing her new tack tries to accomplish. Polls have consistently shown that Palin would fare poorly in a general election. After surveying seven states pitting top Republican candidates against President Obama, the Public Policy Polling group concluded that Palin was “virtually unelectable” thanks to her anemic numbers with Democrats and independents.

A Marist poll last week showed former Massachusetts Gov. Mitt Romney (R) defeating Obama among independents by 8 percent while Palin lost to the president by 17. In Michigan — the state where Palin famously tried “going rogue” in 2008 — Obama leads Palin by 21 percent. And a poll in North Carolina shows Obama keeping the state blue if Palin were the nominee, thanks to the former Alaska governor’s 19 percent favorability rating in that state.

It’s no wonder, then, that Republican insiders have worried she can’t win a general election. Byron York recently noted that even conservative fans of Palin in the pivotal primary state of South Carolina are unconvinced she’s electable.

Perhaps that’s why Palin is suddenly touting her electability. She has to convince those who love her to vote for her, and the way to seal that deal is to persuade Republicans to believe she’s their best shot.