I guess a couple appropriate synonyms might be "blowhard", "full of hot air", or other similarly derogatory terms relating to one's "idle babbling" and "ramblings"...perhaps somewhat like what I am doing even here and now!
OK, I did that on purpose, to prove a point. A "blatherskite" says much but means next to nothing, and in my (humble) opinion, many politicians really do fit into that definition, unfortunately.
I must admit there are some politicians who seem to do their best to avoid appearing like a "blatherskite", and, if they do their homework and understand the people's real needs and desires, and communicate in concise, believable, and otherwise meaningful ways, they can succeed.
Those who truly manage to avoid becoming "blatherskites" might be the ones who ultimately rise to the top, unless the power of Big Money gets in the way and supersedes integrity and honor and greater-good thinking, and turns everyone into 'blatherskites', y'know?