It should also be noted that Nixon had quite an ego, and felt the country really needed him as President, whether it knew it or not.  The cover up of the Watergate break in was his eventual undoing, thanks to the bulldog efforts of Bob Woodward and Carl Bernstein at The Washington Post.  But there were a number of other actions/incidents in that election that were also clearly unconstitutional.  Agents of the CIA disrupted McGovern campaign rallies, sabotaged the electricity, and the Committee to Re-Elect the President (CREEP) spread rumors about McGovern fathering illegitimate African-American children.

The ironic thing is that Nixon didn't need to cheat.  He was pretty popular, had brought most of the troops home from Vietnam already and the economy was still pretty good.  He would have won anyway.

It was absolutely the Watergate break-in which eventually led to Richard Nixon's resignation as President. Nixon had won the 1972 election by a landslide over Democratic opponent George McGovern, so prior to the Watergate scandal, Nixon's popularity was quite high. (I should point out here that I supported McGovern, so not EVERYBODY liked Nixon.) However, after news of the break-in broke, Nixon decided to try and cover-up any involvement within his ranks. Though it is true that Nixon probably knew nothing of the break-in before the burglars were captured, he apparently became enamored with the idea of denying any staff or Republican committee involvement. In doing so, he lied to the American republic and gave the nation the impression that his office was above the law. Nixon eventually lost the support of much of the Republican Congress and Senate, and when impeachment became imminent, he resigned. But before doing so, Nixon apparently made a deal with Vice President Gerald Ford that he would be pardoned of any charges that might follow. For many years, Ford denied that a deal had been made, but this truth was also eventually exposed. 

The break in led to Nixon's resignation because Nixon tried to use his power as president to cover up what had happened.

It does not appear that Nixon personally had anything to do with ordering the break in.  He did not know it was going to happen.  But once it did happen, he thought it could hurt him politically.

Because he thought this, he started using his power as president to try to prevent the facts about the case from becoming known.  This means that he was obstructing justice, which is a crime.

When this got found out, he resigned so he wouldn't be impeached.