Their agenda is to ensure that the least conservative candidate must win. The conservative base and the Tea Party must die, once and for all. Romney is their guy. Since Romney can't get above 25% support among likely Republican voters, the only way to carry him across the finish line is for him to be the only man in the race.
If it were true that the candidates have "noone but themselves to blame for their numbers", we'd be hearing a lot in the media about Romney's "I can't have illegals working for me, I'm running for office!" And we'd be hearing a lot more about ObamneyCare and Romney's extensive, documentable history of flip-flops. We heard a lot about that during the '08 primaries when McCrazy was "their" guy.
The GOP Establishment believes that once Romney is the only guy standing, and all his opposition is irreperably damaged, they can throw us a bone by putting someone like Rubio on the ticket and conservatives will be mollified. They're counting on the conservative fear of an Obama second term to drive us to the polls to pull the lever for Romney.
They're wrong. It didn't work with McCrazy/Palin, and it won't work this time either. In the meantime, if you think they intend to allow Gingrich, or any other viable Romney opposition, to threaten his primary victory, you're going to be in for a shock.