$ 200,000 by Howard Hughes to ensure that there would be no nuclear testing in Nevada.
Nixon took the money and later did not stop the testing - which would have been very difficult to do at that time.
Nixon was paranoid that Hughes was going to take his revenge by leaking the payment to Nixon.
Larry Obrien, Chairman of the DNC had also served as a lobbyist for Hughes. Nixon hoped that he could find information that would neutralize the information that Hughes had about him.
http://en.wikipedia.org/wiki/Watergate Meier told Donald that he was sure the Democrats would win the election because they had considerable information on Richard Nixon’s illicit dealings with Howard Hughes that had never been released, and that Larry O’Brien had the information, <33> (O’Brien who had received $25,000 from Hughes didn’t actually have any documents but Meier claims to have wanted Richard Nixon to think he did). It is only a question of conjecture then that Donald called his brother Richard and told him that Meier gave the Democrats all the Hughes information that could destroy him and that O’Brien had the proof.<34> The fact is Larry O'Brien, elected Democratic Party Chairman, was also a lobbyist for Howard Hughes in a Democratic controlled Congress and the possibility of his finding about Hughes illegal contributions to the Nixon campaign was too much of a danger for Nixon to ignore and O'Brien's office at Watergate became a target of Nixon's intelligence in the political campaign.<35> This theory has been proposed as a motivation for the break-in.