All politicians I've met (bar one) have been very intelligent or at least very cunning when lacking in hard IQ.

What all politicians who run for more than one election term understand: You need to get re-elected. All else stems from that.

The longer versions goes something like this:

1) Change is slow
2) To be able to fuel change, you must keep working at it
3) Due to 1+2, you need to be at it for a long time
4) To be at it for a long time, you need to get re-elected
5) To get re-elected you need to be popular
6) To be popular, you need to echo the sentiments of the voting masses (not minorities)
7) To really compete against others, who are also popular, you need to have a lot of funding to get coverage
8) To get lot on funding, you need to echo the sentiments of those who can fund you
9) Often the sentiments of masses and funders are in direct conflict and you have to betray one or the other
10) When in doubt, betray the masses, because priority no. 1 is to get re-elected and funders have longer memory than voters and in this day and age coverage means more than being popular due to your opinions.

Q.E.D.

And once you do the loop 1-10 plenty of times, it becomes an automaton. It runs by and feeds itself. Pretty soon, it becomes the major goal, not just the sub-goal to help you get things done.

Thus, everything is for sale, except the ability to get re-elected.

There are of course the select few who have strong principles, but they don't usually last for very long in this game, unless their principles happen to be miraculously in-line with both the voters and the funders.