Mitt Romney has been knocked by his Republican opponents as a weak front-runner who can’t lock up the Republican nomination for president, despite his big advantages in money and organization.

His main challengers, Rick Santorum and Newt Gingrich, argue that Romney’s lack of traction stems from a lack of appeal to the party’s conservative base. But exit poll data show Romney’s problem is less about “very conservative” voters than it is with the GOP’s delegate rules this year.

Across all the contests so far, Romney has won 38 percent of the vote to 29 percent for Santorum and 16 percent each for Gingrich and Ron Paul. In fact, Romney’s winning percentage is identical to that of John McCain’s — the eventual 2008 GOP nominee — at this point in the 2008 GOP primaries. And for McCain, that tally includes 15 contests after Romney dropped out after Super Tuesday and two after Mike Huckabee exited the race.

Unlike Romney and Huckabee four years ago, Santorum and Gingrich are sticking it out, offering themselves as the conservative alternative to the moderate Romney. Exit polls show Romney’s outperforming McCain among staunch conservatives.

Averaged across all the contests with exit polls so far, Romney has won 32 percent of the vote among very conservative voters. That compares with 24 percent for McCain four years ago. (Limiting the comparison to 18 states that had exit polls in both 2008 and 2012, Romney is averaging 32 percent to 25 percent for McCain in 2008.) Averaged across contests, Romney runs almost evenly with Santorum among the most conservative voters.

Romney does lag his 2008 performance among these voters. At this time four years ago he had netted an average of 38 percent of the vote across all the states with exit poll data, ahead of Mike Huckabee’s 33 percent. One difference, very conservative voters have made up an average of 35 percent of voters this year across these states, up from an average of 30 percent four years ago.

Romney’s real problem this year may stem from the primary schedule and delegate rules, not a perceived weakness among base voters.

The structure of the delegate allocation process — as reported by Aaron Blake at The Fix — has been slower and more drawn out this time around. So far this year there have been 33 contests, down from 44 over the same period in 2008.

Super Tuesday came a month earlier in the 2008 schedule and had more than twice as many contests on that single day than this year. Through late March 2008, 12 contests were winner-take-all, with McCain winning nine of them for a haul of 432 delegates. This year, there have been only two winner-take-all contests to date, with Romney winning both and netting 79 delegates.

McCain’s delegate wins from those winner-take-all states nearly matches Romney’s current total delegate count of 558. That places Romney about half way to the total of 1,144 delegates needed to secure the nomination. For Santorum to beat out Romney for the nomination he would need to win about 70 percent of the outstanding delegates in the upcoming races, a steep hill to climb.

This time around, Santorum has a little more of a lifeline than did McCain’s challengers after Super Tuesday in 2008. At that point in the contest four years ago, Romney would have needed 80 percent of the remaining delegates to secure the nomination. (One didn’t need an MBA from Harvard to understand those odds.)

The drawn out primary process this year has forced Romney to defend his conservative bona fides, but national polls from the Washington Post and ABC News have found consistently high favorable ratings for him among conservative Republicans. In the latest poll, 66 percent rate him favorably. Pivoting to the general election, Romney’s challenge is less in the base and more clearly among independents where his unfavorable ratings run above his favorables.

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