It used to be easy to predict who the next Republican presidential nominee would be. It was decided by primogeniture: The next oldest guy in line got to be the king.

[IMGCAP(1)]It’s not so easy looking to 2012, with former Vice President Dick Cheney out of the running and a woman, soon-to-be former Alaska Gov. Sarah Palin, in.

And I do believe she’s in — damaged in her chances, maybe, but fully intending to make a run and very popular with the shrinking hard core of the GOP.

In the Democratic Party, primogeniture sometimes applies, as with Adlai Stevenson in 1956 and incumbent or former Vice Presidents Hubert Humphrey in 1968, Walter Mondale in 1984 and Al Gore in 2000.

But, just as often, a candidate will come from relative, or not-so-relative, obscurity and seize the nomination, such as John F. Kennedy in 1960, George McGovern in 1972, Jimmy Carter in 1976, Michael Dukakis in 1988, Bill Clinton in 1992 and Barack Obama in 2008.

In the Republican Party, however, there’s a rigid history of “follow the leader.— When Dwight Eisenhower’s second term was over in 1960, the nomination went to his vice president, Richard Nixon.

Nixon lost, of course, and didn’t run in 1964. So two next-in-lines ran against each other, conservative Sen. Barry Goldwater (Ariz.) and New York Gov. Nelson Rockefeller — and Goldwater won, then lost, disastrously.

Nixon was back in 1968. His Watergate-installed successor, Gerald Ford, survived a challenge from Ronald Reagan in 1976 to be the nominee. Then Reagan won in 1980, after which his vice president, George H.W. Bush, became king.

After Bush lost in 1992, Sen. Bob Dole (Kan.), Ford’s 1976 running mate, won the 1996 nomination. Arguably George W. Bush’s ascendancy in 2000 was a break with primogeniture. But his name was Bush, after all.

Even in the 2008 election, Sen. John McCain (Ariz.) was the next-in-line guy, having run against Bush in 2000.

Palin, as McCain’s running mate, had every chance to have primogeniture working for her.

Even though she demonstrated an utter lack of qualification to be a heartbeat away from the presidency last year, all she had to do to become the party’s clear frontrunner was knuckle down, pick smart advisers, study hard, travel abroad, make serious speeches and raise lots of money for the party.

Instead, she’s become a running soap opera, what with disputes between her advisers, doubts about what events she’ll show up at, public spats with the teenage father of her out-of-wedlock grandchild and nonstop battles with the media and her other critics.

Has she been savaged? She has. Clearly Palin-hatred is rampant in the media, the left-wing blogosphere, the Democratic Party and parts of the McCain campaign staff.

But now comes her decision, not just to pass on running for re-election in 2010, but to quit the Alaska governorship with 18 months left in her first term.

The manner of her pulling out was weird, but the decision itself has logic. Instead of dealing with ethics probes and legislative fights in far-off Juneau, she’s now free to make money for herself and the GOP, pay off her legal bills, roam the lower 48 and get lionized at right-wing rallies.

And, run for president. Clearly, she intends to try it.

In her resignation announcement, she said there is “a need to build up and fight for our state and our country. I choose to fight for it!—

“And I’ll work hard for others who still believe in free enterprise and smaller government; strong national security for our country and support for our troops; energy independence; and for those who will protect freedom and equality and life.—

That might sound like a pledge just to help fellow conservatives, but in a Facebook message on Sunday, she compared herself to other officials who have left office early “for a higher calling— — presumably, Obama — without being criticized.

Does she have a chance to get nominated? You betcha. She’s attractive, charismatic, ambitious, tough to the point of ruthlessness and smart, if still woefully ignorant.

Before she announced her resignation, she was tied in a CNN poll with 2008 candidates Mitt Romney and Mike Huckabee and a Pew poll showed that she had an 85 percent approval rating among conservatives and white evangelicals, to 52 percent for Romney.

My bet is that primogeniture still works — sort of — and that Palin, Romney or Huckabee, familiar faces, will outrun national newcomers like Minnesota Gov. Tim Pawlenty and Indiana Gov. Mitch Daniels.

Let’s hope, as citizens, that all the GOP candidates including Palin get serious about devising workable conservative solutions to America’s problems and don’t just figure out how to pander to the party’s right-wing base, as Romney did for most of 2008.

Here’s a scary scenario for you, though: Obama, for all his talent, fails to revive the economy, overburdens the country with debt and blunders in foreign policy, making himself deeply vulnerable in 2012.

And his opponent is just a slogan-shouting darling of the right, male or female. What a choice! So, let us hope that the Republican race becomes a battle of ideas.