In unfinished tails it elaborates on this. One of the reasons(besides his strength), according to UT, that he can resist Sauron is because he is the rightful user of the Palantir and that gives him an advantage somehow. It is similar with Denethor since he is acting for the king, however the advantage is not as strong. Saruman has no claim on the stones, and thus, he is more easily ensnared.
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