E.J. Dionne writes that Mitt Romney’s greatest asset — his perceived electability — may have disappeared, in his column, “Romney’s Big Problem:”

Mitt Romney has lost his central asset. It is no longer obvious that he is the Republican with the best chance of defeating President Obama.

Romney was never fully trusted or liked by the staunchest conservatives, a rather large Republican constituency. But until now, enough of them have been willing to swallow their doubts at critical moments because they believed the former Massachusetts governor was the one potential nominee who could win the election.

This is not true anymore. Reflecting the damage Romney’s image has suffered in the six weeks since voting started in Iowa, he is running little better than Rick Santorum, now his main opponent, in matchups with the president. And both of them are losing.

In the latest New York Times/CBS Poll, released on Tuesday, Romney was behind Obama by six points, while Santorum trailed by eight — a margin-of-error sort of difference. By contrast, the struggling Newt Gingrich trailed by 18 points. Similarly, a recent Pew Research survey found Romney behind Obama by eight points, Santorum was losing by 10, and Gingrich by 18. And a Public Policy Polling survey actually found Santorum running a net two points better than Romney against the incumbent.

As long as Gingrich was his main competitor, Romney had a potent electability argument. But Santorum can say that since he is more or less equally strong against the president, conservatives might as well vote their convictions and their hearts.