Why the GOP nominee won't pick Palin or Santorum. Why he should pick Rubio. By David Morris, Deputy Managing Editor April 3, 2012 When Mitt Romney gets around to picking his running mate, one thing will matter more than anything else: The person's ability to fire up conservatives in the Republican base. SEE ALSO: Why Rubio Is Right for RomneyIt's no secret that many conservatives -- a clear majority of them in most of the states that have voted so far -- aren't happy with Romney and would love to see the party pick someone else to run against President Obama. But that won’t happen, so look for Romney to pick someone far more conservative than he is. Cross Rick Santorum off the list, though. His decision to stay in the race even when Romney's nomination became inevitable -- a course that was sealed April 3 in Wisconsin -- leaves too much bad blood for Romney to ask. Advertisement For that matter, forget anybody who was in the GOP race at some point this year: Newt Gingrich. Rick Perry. Michele Bachmann. Herman Cain. Jon Huntsman. Ron Paul. Tim Pawlenty. Some of them ticked off Romney. Others don't bring enough to the table to make a difference. I'd put 2008 GOP vice presidential nominee Sarah Palin in the no-go category, too. There's no doubt she'd help Romney among conservatives, and some see her as the shot in the arm Romney needs to force Obama into early retirement. But she is such a lightning rod for criticism that she'd also scare off some moderates and independents who otherwise might be comfortable with Romney's candidacy. Don't be surprised when most, if not all, of the following names show up on Romney's short list as he works through the selection process in advance of the GOP convention in Tampa, Fla.: Marco Rubio. The senator from Florida can help Romney across the board -- with the tea party, with Hispanics and in the state itself, a must-win for the GOP. He's the best bet, even though he says he isn't interested. Though he and Romney differ on immigration policy, Rubio's spot on the ticket could sway enough Hispanic voters for Romney to carry Florida, hold on to Arizona and make it close in New Mexico. Add Texas, which Romney will win no matter whom he picks, and the GOP has a shot at toppling Obama. Just a few percentage points among Hispanics could mean the difference between winning and losing. For example, Republican George W. Bush won the presidency in 2000 with 35% of the Hispanic vote, while John McCain lost in 2008 with 31% of Hispanics. Advertisement Susana Martinez. The governor of New Mexico, a former prosecutor, has solid conservative credentials and Hispanic roots. She also could help Republicans make gains among women. Her presence on the ticket might not be enough to wrest New Mexico from Obama's column, but it would be the year's biggest surprise. Bob McDonnell. Virginia's governor could prevent Obama from carrying the state for the second time. He is a conservative, but not nearly as well known as some of the other names likely to pop up on Romney's list. Chris Christie. The New Jersey governor is the top choice of many GOP insiders. But he's not as conservative as many in the party believe. He favors some gun control laws, calls for a "clear path" for illegal immigrants to become citizens and expresses the belief that climate change is a serious issue. In addition, he has zero chance of delivering New Jersey for Romney. Paul Ryan. Romney would mend a lot of fences with conservatives by choosing the chairman of the House Budget Committee. But while Ryan scores with the base in many parts of the country, he just might deliver Florida to Obama. The reason: Ryan's view that Medicare needs to be changed as part of a broad effort to control the deficit won't sit well with Florida's senior citizens. Advertisement Mitch Daniels. Republicans of all stripes clamored for the Indiana governor to run for president, but he opted out because his family objected. It's hard to see those objections softening so he can take the second spot. Plus Daniels has some serious general election baggage: He was the chief architect of many of President George W. Bush's spending policies, which will be in Obama's crosshairs in November, and which even some Republicans say contributed to the nation's deficit mess. Rubio is Romney's best bet. But if Rubio sticks to his guns and doesn't run, look for one of the other five to emerge.