Rand is basically conceding that it is most important to elect a Republican — any Republican — as president and is willing to do whatever it takes to accomplish that end. The senator’s endorsement cited his discovery of considerable common ground with Romney, something his father never managed to achieve.
Daniel McCarthy, by contrast, endorses it, contrasting Rand's "integrationist" strategy for changing the GOP with Ron's "party-within-a-party" approach:
It’s possible that the integration strategy will backfire, that by lending support to Romney it will lend support to the neocons eager to occupy his administration and who already fill the ranks of his foreign-policy advisers. But the attempt to build a third party has comprehensively failed, as the sorry annals of the Libertarian, Constitution, and Reform parties show, and the attempt to build a party-within-a-party showed no signs this year of having a chance. If the integration strategy fails — and giving up one’s principles would be the greatest failure of all, yet so far Rand’s Senate record is pretty good, and [Pat Buchanan] never surrendered his realism or economic nationalism for the sake of Bush or Dole — then the liberty movement will have to try something else. There’s no a priori path to succeeding in politics in order to change policy in this country; like many things, political strategy is a matter of trial and error.