If we define ‘lying’ as not being completely open about your intentions, then I don’t see how you can be a politician without lying.
Some interests are shared – just about everybody wants a prosperous economy, and protection from terrorism.
But on, say, energy prices, it’s a little more complicated. My home state of West Virginia had a long history of seeing its coal-heavy economy improve when energy costs were high, which often coincided with recessions in the rest of the country.
A politician needing the votes of two such competing factions sometimes survives via a delicate dance around the conflict.
E.g.: A Republican candidate is running for office in a highly-unionized region. Because Republicans have not historically had the best relations with organized labor, he makes vague promises to support labor. He wins the election, and after taking office, does some things that aren’t all that good for labor.
A pro-union person who voted for the politician would likely call him a liar.
But an anti-union person would be more inclined to chuckle, “Heh, heh – good one, buddy”, and defend the pol’s actions by pointing out that he never promised not to commit these specific actions.
When is a politician lying?
The answer often depends on whether you think the politician is lying to you, or to someone whose interests oppose yours.
(Incidentally, I don’t mean to pick on Republicans. My example was based loosely on discussions I heard in the wake of Ronald Reagan’s firing the striking PATCO air traffic controllers. I’m sure there are instances of Democrats doing the same thing, say, when running for office in staunchly pro-gun regions).