The ironic thing about it was that, after Watergate and Ford's pardoning of Nixon, much of the public was pretty miffed at the Republicans. The Democrats had an opportunity to capitalize on the situation. In 1976, Reagan was considered a ridiculous joke, just a senile and dumbed-down version of Nixon.
I thought Carter was a decent President, but perhaps too decent for the Washington crowd. What was even more disappointing is that many Democrats went against him and made his presidency so much more difficult, even though Democrats controlled both houses of Congress. The Democrats were in the catbird seat, and then self-destructed all on their own. The turmoil in Iran and the Shah was definitely completely botched by the Administration, along with the embassy hostage crisis and the failed rescue mission. Ted Kennedy threw Carter under the bus and wrecked the Democrats' chances in 1980.
The Iranians pulled off a major coup in causing a resurgence in American warmongering. After Vietnam, Americans were more and more focused on peace and global responsibility, but thanks to the Iranians, who played right into Reagan's hands, they put Reagan in the White House and started a new era in American militarism and imperialism. The Iranians made war cool again in the eyes of millions of Americans. After that point, the peace movement was associated with weakness, cowardice, and wimpiness. Michael Dukakis tried to counter those criticisms by showing that he's tough, by riding in a tank.
Clinton also had a golden opportunity in 1992 to make a real difference, both in domestic policy and in foreign policy, but instead, he chose to support NAFTA and free trade, and continue the status quo of US foreign policy - even though the Soviet Union had disbanded and the Cold War was over. Clinton clearly did not understand the outside world, and he totally botched US foreign policy and US trade policy. 9/11 could also be indirectly blamed on Clinton's incompetence, since the hijackers entered America under Clinton's watch. But his record showed that he was a warmonger just the same, yet too incompetent to defend American territory or America's interests. That seamlessly segued into Bush's presidency, which seamlessly segued into Obama's presidency. Absolutely no change in policy for those three two-term Presidents, who should probably be known as the "Three Stooges." Clinton and Bush were interchangeable, and Obama was merely Bush Lite. We could have had no President and just let the government run on automatic and gotten the same result.
My main frustration at this point is that no one can even see it. Democrats still seem to think that they can do no wrong. They won't admit that any of the policies have been flawed. They won't admit that they've been out of touch with the needs of the people. They think that they're above reproach, perfect and flawless, and they spew venom at anyone who thinks they're not (as we've already seen in some of the comments in this thread).