Why is defying majority support for gun control "cowardice"?

In Giffords’ view, these senators are two-faced, because you cannot truly sympathize with her unless you vote for the bills she supports. But I am a little confused about the purported motivation for this perceived betrayal. Obama and Giffords both insist the senators who voted against new gun controls did so not out of conviction but out of fear—specifically, fear that they would be defeated the next time they run for re-election. If their constituents “overwhelmingly favored expanding background checks,” however, wouldn’t voting for the bill mandating those have been the politically expedient thing to do? And why is opposing the will of the majority a mark of “cowardice,” as Giffords says, rather than a mark of courage?

Furthermore, why would senators be afraid of “the gun lobby” unless they think it can sway voters against them? Isn’t that ultimately the source of the NRA’s fearsome power? But if voters are so easily manipulated, why should we be impressed by majority support for expanded background checks or any other gun control measure? I suspect that Giffords credits the majority with wisdom only when the polls are going her way, just as she credits politicians with integrity only when they agree with her.