The Gospel writers tell us that there was a great interest in what John was doing. In fact, even Josephus speaks of his ministry and says some Jews reasoned that the later defeat of Herod Antipas’ army to Aretas, king of Arabia, was evidently due to God judgment upon him for his killing John. So, John was a force to be reckoned with, at least according to Josephus, who records that Herod feared John might use his popularity to raise a rebellion against him. The Gospel accounts show John’s public ministry ended with his imprisonment, specifically on charges of John claiming Herod was living in sin, because he had married his brother, Philip’s, wife, Herodias (Luke 3:19-20). Herod probably had John beheaded within a year of his arrest.
People came to John’s baptism from Jerusalem, all Judea and regions around the Jordan river (Matthew 3:5). They came from all walks of life—the general public (Luke 3:10), publicans (tax collectors), soldiers (Luke 3:12, 14) and harlots (Matthew 21:31-32). They came and repented, but people like Herod and the leaders of the Jews at Jerusalem were suspicious of him.
The Pharisees and Sadducees came to John’s baptism, but I hardly think sincerity was their motivation. Although some may have come because they were pricked in their hearts, they, nevertheless, seem to have wanted to avoid repenting of their responsibility for the errors of their nation. However, as I said earlier, the Pharisees and Sadducees (especially the leadership at Jerusalem) wished to infiltrate John’s ministry in an effort to control or destroy it.
Earlier (John 1:19-22), the chief priests (Sadducees) sent priests and Levites (Pharisees according to John 1:24) to John asking who he was claiming to be. Was he the Messiah? Was he Elijah who was predicted to come near the time of the Messiah? Was he that Prophet that Moses predicted would come that would have authority like Moses had (cf. Deuteronomy 18:15-18), but John denied being any of these (John 1:20-21).
The sect of the Pharisees believed the Oral Law was binding upon all. This law contained a variety of washings (Mark 7:3-7) and other laws and the Pharisees claimed that it came down to them from Moses, orally, through the traditions of the fathers. It was in defense of their doctrine that they disputed with John’s disciples over the idea of purifying (John 3:25). Their contention had to do with cleansing. The traditions of the elders had already called for multiple cleansings or immersions (baptisms) in running water at Jerusalem. Why was John’s baptism needed at all? What was he claiming to do that wasn’t already being done by the authorities at Jerusalem. John’s ministry implied either insufficiency of or the corruption of Jerusalem’s priesthood. Either way, Jerusalem authorities frowned upon what John was doing.
John’s baptism of repentance seemed to them to conflict with the need of the many washing having to be done under the Oral Law. John simply commanded folks to repent or turn back to God and witness to their inner commitment by submitting to an outward cleansing or baptism. This put the Jewish authorities and John at an impasse. Either the authorities had to repent and reject the traditions of the fathers or John and his baptism had to go.
Therefore, the interest in John’s ministry was wide and varied. Not everyone who came to him was sincere, but many were, especially the common folk, the soldiers, tax collectors and the harlots. Nevertheless, the leadership, whether political or spiritual felt threatened by John’s popularity and wondered if he might raise a rebellion—against Herod (perhaps to have Rome replace him) or against the Jerusalem authorities to replace them.
 Josephus, Antiquities of the Jews (18.6.2).