Santorum and Ron Paul don't share the same base. Or even remotely similar bases (except, apparently, in Iowa). My bet is that less than five percent of people who would have voted for Santorum head to Ron Paul. Furthermore, Santorum dropping out a month or so ago would have handed Romney the nomination.
Santorum is dropping out because a loss in his home state kills any chance he had of another political run (maybe in 2016, maybe in Penn for some other office). More charitably, I am sure his daughter's illness and hospitalization had something to do with his choice as well.
Anyways, this is probably the end of the primaries. Santorum supporters are likely to just not vote if they really hate Romney, and now that his biggest opponent is out of the race Romney will probably take the rest of the delegates in a landslide.
Those Santorum voters were probably not going to vote anyway by now, because of sensing Romney was inevitable, even if Rick stayed in. The absense of Santorum from the race is a null factor at this point, as his real impact on the primaries was to act as a shunt to deposit non-Romney votes that might have otherwise have gone to Paul. If Newt and/or Rick had dropped in January when they knowingly lacked the resources to run a national race against Romney, Paul would have consistently gotten about 40% or more of the primary or caucus vote, and certainly would have won several of the beauty contests, and upfront would have won a larger apparent amount of delegates in the proportional states.
That performance, translated to 40% or more of the apparent delegates, could have given Paul the momentum to easily frustrate Romney from getting the 1144 needed to clinch. This is why the establishment made sure Newt and Rick stayed in to run resistance for Mitt. The point was not merely to deny Paul from winning states outright,
but to stop the only nationally organized opposition to Romney from ever getting consistent momentum and media coverage. The use of the lackluster candidates who could not win nationally was to provide the MSM cover for never treating Paul like a first tier threat to Mitt (and ultimately, to black him out altogether). It allowed reporters to assert "of course we're not covering Paul seriously, hey, he can't win a state!" and other self-fulfilling laments.
Now that their primary job in robbing Paul of vote victories, momentum and top tier media coverage is over, as Romney now has a formidable lead going into mostly winner take all states, Santorum and Gingrich are quitting on cue or reconciling themselves with Mitt, just as I (last month) predicted they would by this point. The tycoons who 'suddenly' emerged in January to breathe financial life into the two's cashless candidacies have similarly just as 'suddenly' shut their wallets, indicating their role in protecting Romney from Paul by funding Newt/Rick was also orchestrated.