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A year from now, the polls tell us, Joe Biden, Elizabeth Warren or maybe even Bernie Sanders will spoil Donald Trump's bid for a second presidential term by a decisive margin. Maybe.

While that might prove to be the 2020 scenario, history tells us there's a good chance next year's election will look a lot different from the way it does now. That could mean almost anything from Trump's re-election to a narrow Democratic victory by some other nominee over Vice President Mike Pence.

A prime example was the 1980 election that chose Ronald Reagan. A year earlier, he not only trailed President Jimmy Carter, but was even further behind Massachusetts Sen. Ted Kennedy. And Kennedy seemed likely to deny Carter renomination.

The conventional wisdom was that Reagan was too conservative to be elected; certainly, the Carter folks felt that way. Kennedy faded, and, as late as three weeks before that election, polls showed Carter ahead. But the former movie star turned California governor wound up winning a 44-state landslide.

The only other recent president to lose re-election, George H.W. Bush, looked like such a good bet a year out that many top Democrats opted against running, leaving a field of lesser-known candidates.

One was Bill Clinton, whose support was in single digits in November 1991, well behind Mario Cuomo, until the New York governor decided against running. The next year, the Arkansas governor lost both the Iowa caucuses and the New Hampshire primary, had to surmount not one but two scandals and still beat Bush (and independent Ross Perot).

All three presidents since then — Clinton, George W. Bush and Barack Obama — were reelected. But their success was not at all clear one year out.

Where things have turned out most differently has been in the nominating battles among non-incumbents, especially in the party out of power. Here are some:

• 2016: By November, Trump already led GOP polls, despite widespread doubts over his prospects. But he consistently trailed Hillary Clinton in general election matchups, and even his own people were surprised when he won.

• 2012: At various times in the fall of 2011, ultimate GOP nominee Mitt Romney trailed former House Speaker Newt Gingrich, Texas Gov. Rick Perry and business executive Herman Cain.

• 2008: Obama entered the Democratic race with substantial fanfare. But in the fall of 2007, he trailed Hillary Clinton (while ultimate GOP nominee John McCain was losing to former New York Mayor Rudy Giuliani). Obama's Iowa caucuses victory reversed that, and his South Carolina primary triumph solidified his lead.

• 2004: Before Massachusetts Sen. John Kerry finally emerged as President George W. Bush's opponent, the Democratic race had multiple leaders: Connecticut Sen. Joe Lieberman, the Party's 2000 vice presidential nominee; Gen. Wesley Clark, helped briefly by opposition to the Iraq war; and former Vermont Gov. Howard Dean, one of the first to tap the internet as a fundraising tool. But a nasty Iowa ad war between Dean and Missouri Rep. Dick Gephardt sank both, and Kerry grabbed the lead.

• 1988: The early favorite, Colorado Sen. Gary Hart, withdrew after disclosure of the married senator's weekend cruise to an offshore island with a young woman. At first, civil rights icon Jesse Jackson took the lead in a large field. Eventually, Massachusetts Gov. Mike Dukakis became the nominee.

• 1976: Jimmy Carter's election was certainly the biggest modern electoral surprise until Trump.

Early Democratic polls showed a group bunched at the top: former Alabama Gov. George Wallace; Washington Sen. Henry Jackson; and former Vice President Hubert Humphrey, who never actually became a candidate.

Carter was well back, in single digits. But when the former Georgia governor won both Iowa and New Hampshire, he took a lead he held the rest of the year, despite heated opposition from, first, the Democratic establishment and, later, GOP President Gerald Ford.

In the end, Carter sneaked in by a total margin of about 18,000 votes in Hawaii and Ohio, even narrower than the 77,000 votes by which Trump unexpectedly won Michigan, Pennsylvania and Wisconsin in 2016.

One wild card in any election is the fact that, in the preceding 12 months, anything can happen and often does, changing the foreordained electoral dynamic.

In November 1979, exactly one year before the 1980 election, Iranian militants invaded the U.S. Embassy in Tehran, detaining nearly 70 American diplomats. At first, Carter's tough rhetoric buoyed his political standing, but his support dropped as months passed and he was unable to free them, including an ill-fated helicopter rescue mission.

Three years after his election, President Reagan was beset by a lingering economic recession and consistently trailed Carter's vice president, Walter Mondale.

But the recession ended, the economy rebounded, and Reagan abandoned some of his tough rhetoric toward the Soviet Union. Slowly, the polls turned around, and, on Election Day 1984, Reagan carried 49 of the 50 states, one of history's biggest electoral landslides.

It's a scenario Trump would love to duplicate.