Well, the clear answer here is that it ended Nixon's career as a political figure.  It ended his presidency 2 years before it was going to end anyway, but it also pretty much ruined his reputation and his ability to act as any sort of an "elder statesman" after his presidency ended.

Had it not been for Watergate, I think Nixon would have had a much different career after he left office.  He would have been seen as the guy who opened up diplomatic relations with China and who got the Vietnam War ended.  I think that he would have been able to have a career more like that of Jimmy Carter who has become a major player in various international affairs after he left office.

The Watergate Scandal has defined Richard Nixon's legacy as a former President.  Despite the fact that he was elected twice, served six full years and successfully extracted us from an unpopular, protracted and expensive war, he will always be remembered for his spectacularly bad judgement regarding this scandal.

Not that I feel this is undeserved, it was a criminal act by a President who had come to believe that he was above the law.  He deserved impeachment had he not resigned, but as a historian, I have to acknowledge there were other important and positive things he did during his Presidency.

The Clean Air and Water Acts were passed on his watch, and he aggressively pushed for integration of schools through busing, which was unpopular at the time, even if socially necessary.  He successfully drove a wedge between two adversaries, the Soviet Union and China through detente, and locally, where I live, he gave Mt. Adams to the Yakama Indian tribe as sacred native land.

Despite all of this and anything else he may have achieved, he will always be, first and foremost, associated with the Watergate Scandal and the other unconstitutional acts that came before and after it.

How did the Watergate scandal affect the reputation of President Nixon?

Simply put, Watergate completely destroyed Nixon's reputation. For many years, political opponents had alluded to his penchant for dirty tricks, but few people thought he would stoop so low as to break the law and violate the Constitution he had sworn to uphold.

Nixon had been reelected in 1972 in a massive landslide victory over his Democratic opponent, George McGovern; he had achieved a notable foreign policy success in establishing good relations with Communist China. Yet, Nixon's paranoia and chronic insecurity meant that he could never feel truly comfortable in his own skin. In particular, he believed that the press were uniformly hostile and out to get him. Even though he had just roundly defeated the Democrats in the race for the White House, he still did not hesitate in authorizing illegal wiretaps to be used against them.

Nixon's reputation was destroyed by Watergate for two main reasons. First, the President had clearly broken the law. The head of state, despite Nixon's loud protestations to the contrary, was a criminal. Secondly, he not only authorized illegal acts against his opponents, he actively tried to cover them up. Nixon's long-standing reputation for dishonesty now began to catch up with him. Even those who had overlooked his duplicitous behavior in the past realized that this was a much more serious and much more dangerous situation.