Sen. John McCain, who won the Republican nominee for president in the 2008 election, has largely avoided commenting on the 2012 field of candidates. But he’s gearing up to finally endorse one of them. Sources tell The Hill it won’t be Newt Gingrich.

Republican sources familiar with his thinking say he will probably endorse his onetime political enemy, Mitt Romney.

There is virtually no chance, sources say, that he will pick former House Speaker Newt Gingrich (R-Ga.), who has surged into the lead in polls, stoking consternation among his ex-colleagues on Capitol Hill.

The reason: the same one that keeps coming back up. Electablity.

There is growing concern among many of McCain’s Senate Republican colleagues on whether Gingrich is electable in a match-up against President Obama.

“Newt’s hand is always six inches from the self-destruct button,” one GOP lawmaker said last week.

McCain said, “I don’t know if he’s electable. You have to see how candidates evolve.”

Also, Romney endorsed McCain in his re-election bid for a fifth term in the Senate back in February.