I'll do 1960 - 2012:
1960:
John F. Kennedy (D) 51%
Richard Nixon (R) 48%
Explanation: Kennedy was a decisive leader and copied the GOP on tax cuts.
1964:
Barry Goldwater (R) 55%
Lyndon B. Johnson (D) 44%
Explanation: Vietnam.
1968:
Hubert Humphrey (D) 49%
Richard Nixon (R) 46%
George Wallace 4%
Explanation: Vietnam.
1972:
Richard Nixon (R) 57%
George McGovern (D) 42%
Explanation: Despite Watergate, McGovern was still too liberal for the country.
1976:
Gerald Ford (R) 60%
Jimmy Carter (D) 39%
Explanation: Despite Watergate, high inflation and unemployment plus a weak foreign policy are not popular.
1980:
Ronald Reagan (R) 64%
Jimmy Carter (D) 35%
Explanation: Carter's unpopular presidency, plus Reagan's popular one.
1984:
Ronald Reagan (R) 61%
Walter Mondale (D) 38%
Explanation: Despite Iran-Contra, Reagan's second term saw more economic growth and the end of the Cold War.
1988:
George Bush (R) 52%
Michael Dukakis (D) 47%
Explantation: Michael Dukakis ran a terrible campaign and was not a successful Governor, but the economy did see recession in the early 1990s.
1992:
Bill Clinton (D) 40%
George Bush (R) 36%
Ross Perot (I) 23%
Explantation: Washington became divided in 1993 and 1994, and having an independent as President would have been more popular than it was. Clinton still would have won, as people wanted a change.
1996:
Bill Clinton (D) 53%
Bob Dole (R) 42%
Ross Perot (Reform) 4%
Explanation: The great economic growth of Clinton's second term canceled out any anger towards his personal behavior.
2000:
George W. Bush (R) 53%
Al Gore (D) 46%
Explanation: George W. Bush's tax cuts led to the shortest recession in decades, his foreign policy and homeland security policies were popular during his first term, and Al Gore was a sore loser.
2004:
John Kerry (D) 51%
George W. Bush (R) 48%
Explanation: Iraq & the financial crisis
2008:
John McCain (R) 51%
Barack Obama (D) 48%
Explanation: Obamacare & the continuation and even deepening of the great recession. McCain would be at 53% or 55% had he picked a more popular running-mate.
2012:
Mitt Romney (R) 57%
Barack Obama (D) 42%
Explanation: Romney being proven right in foreign affairs, continuing economic woes, dysfunction in Washington, Obama's executive orders, and Obamacare.
