After watching clips last night of Mitt Romney promising to defund Planned Parenthood and attacking Santorum for insufficient zealotry in matters sexual, I found myself engaged in a thought experiment. The analogy is imperfect, but the point of such things is just an imaginative nudge, so here goes.
Imagine that Joe Manchin, formerly governor of West Virginia and currently second most conservative Democrat in the Senate by voting record, runs for president in 2016 -- a year in which Democrats are frothing after four years of massive tax cuts and spending cuts (and maybe a new war or two) under President Santorum. In his Senate tenure, Manchin has voted against cloture for the bill repealing DADT, voted for the Blunt amendment that would have exempted any employer from providing coverage for any treatment that the employer objects to on grounds of personal conscience, voted against the payroll tax cut extension, announced that he would vote against raising the debt ceiling without a deficit reduction bill, and currently calls for "reforming" the individual mandate that is the lynchpin of the Affordable Care Act.
Now suppose the Joe Manchin of 2016 promised Democratic primary voters that he would support a Constitutional amendment legalizing gay marriage, seek to repeal the Hyde Amendment barring use of Federal funds to pay for abortion, pursue a deficit reduction deal that would raise $5 trillion in new revenue over ten years, and add a strong public option to the Affordable Care Act. Suppose too that his chief pitch to the Dem base was that as a severely moderate Democrat he would be more electable than his garden variety liberal rivals.
Not only is Romney not credible as a "severe conservative": his pretending to be one undercuts his electability argument. Nixon famously advised Dole to run to right as hard as he could in the primary, and run back to the center as fast as he could in the general. But you can't run both ways at once.