Actually most politicians do not break promises all the time, people just say that like a mantra, plus individual people tend to decide something was a promise that wasn't and wasn't meant to be. This is also a written document they have to read and sign with short and clear language, and it has been made abundantly clear since the pledge's inception that violating the pledge, particularly its spirit or in a major fashion, would all but guarantee a strong primary challenge. I would consider voting for someone who didn't sign the pledge, for instance, but I would not vote for someone who signed it and broke it in any tangible fashion. There is some wiggle room for raising taxes in the language too, nor is it a suicide pact, but and I think most GOP primary voters would want a damn good reason for the violation, here's the House of Reps pledge:

I, _______________, pledge to the taxpayers of the _____ district
of the state of__________, and to the American people that I will:
ONE, oppose any and all efforts to increase the marginal income tax
rates for individuals and/or businesses; and
TWO, oppose any net reduction or elimination of deductions and
credits, unless matched dollar for dollar by further reducing tax rates.

On the campaign trail there is no room for a violater to say "That was out of context" or "This is a nuanced issue". And that's been signed by all but 6 of the GOP House Reps as well as 2 Dem house reps. I don't believe its really been specified but a house rep who wanted out of the pledge could say "I'm out of the pledge after this term" and so long as there was another primary election for them before they opted out I don't think there'd be an accusation of breaking their pledge. Other than that, they'd have to have some really, really good extenuating circumstances or face a powerfully backed primary challenger. This is a written pledge, not some short comment on the campaign trail, most voters would frown strongly on violating something like that and if anything the staunch GOP loyalists who tend to cough up donations and routinely vote in primaries tend to be even more hardcore about violating what would be seen as a written contract. So, every time the democrats try this again with some minor alteration, it's a non-starter and they know it, which is why we accuse them of playing politics rather than making serious suggestions. That is why we keep calling it a non-starter, it requires from the get-go that someone break their word.