The stage is set prior to the first presidential debate at Magness Arena at the University of Denver in Denver, Colorado, October 3.
AFP/Getty Images
The stage is set prior to the first presidential debate at Magness Arena at the University of Denver in Denver, Colorado, October 3.
Now playing
02:06
Best debate moments in history
The stage is set prior to the first presidential debate at Magness Arena at the University of Denver in Denver, Colorado, October 3.
AFP/Getty Images
The stage is set prior to the first presidential debate at Magness Arena at the University of Denver in Denver, Colorado, October 3.
Now playing
02:42
14 months of debates in under three minutes
Democratic presidential nominee former Secretary of State Hillary Clinton and Republican presidential nominee Donald Trump speak during the town hall debate at Washington University on October 9, 2016, in St. Louis.
Win McNamee/Getty
Democratic presidential nominee former Secretary of State Hillary Clinton and Republican presidential nominee Donald Trump speak during the town hall debate at Washington University on October 9, 2016, in St. Louis.
Now playing
07:01
7 fact checks from the town hall presidential debate
ST LOUIS, MO - OCTOBER 09:  Republican presidential nominee Donald Trump listens during the town hall debate at Washington University on October 9, 2016 in St Louis, Missouri. This is the second of three presidential debates scheduled prior to the November 8th election.  (Photo by Scott Olson/Getty Images)
Scott Olson/Getty Images
ST LOUIS, MO - OCTOBER 09: Republican presidential nominee Donald Trump listens during the town hall debate at Washington University on October 9, 2016 in St Louis, Missouri. This is the second of three presidential debates scheduled prior to the November 8th election. (Photo by Scott Olson/Getty Images)
Now playing
01:39
Trump vs. the moderators at the town hall debate
trump clinton 60 seconds vitriol origwx js_00000000.jpg
trump clinton 60 seconds vitriol origwx js_00000000.jpg
Now playing
01:00
60 seconds of pure vitriol from Sunday's town hall debate
Democratic presidential nominee former Secretary of State Hillary Clinton and Republican presidential nominee Donald Trump speak during the town hall debate at Washington University on October 9, 2016, in St. Louis.
Win McNamee/Getty
Democratic presidential nominee former Secretary of State Hillary Clinton and Republican presidential nominee Donald Trump speak during the town hall debate at Washington University on October 9, 2016, in St. Louis.
Now playing
02:03
The scorched-earth presidential debate in 2 minutes
vice presidential debate mike pence v donald trump origwx allee_00000000.jpg
vice presidential debate mike pence v donald trump origwx allee_00000000.jpg
Now playing
02:14
Actually, Mike Pence, Trump really DID say that
HEMPSTEAD, NY - SEPTEMBER 26:  Republican presidential nominee Donald Trump stands at his podium during the Presidential Debate at Hofstra University on September 26, 2016 in Hempstead, New York.  The first of four debates for the 2016 Election, three Presidential and one Vice Presidential, is moderated by NBC's Lester Holt.  (Photo by Win McNamee/Getty Images)
Win McNamee/Getty Images
HEMPSTEAD, NY - SEPTEMBER 26: Republican presidential nominee Donald Trump stands at his podium during the Presidential Debate at Hofstra University on September 26, 2016 in Hempstead, New York. The first of four debates for the 2016 Election, three Presidential and one Vice Presidential, is moderated by NBC's Lester Holt. (Photo by Win McNamee/Getty Images)
Now playing
02:37
5 times Trump was live fact-checked at the debate
(L-R) Republican presidential nominee Donald Trump and Democratic presidential nominee Hillary Clinton shake hands prior to the start of the Presidential Debate at Hofstra University on September 26, 2016 in Hempstead, New York.  The first of four debates for the 2016 Election, three Presidential and one Vice Presidential, is moderated by NBC's Lester Holt.
Drew Angerer/Getty Images
(L-R) Republican presidential nominee Donald Trump and Democratic presidential nominee Hillary Clinton shake hands prior to the start of the Presidential Debate at Hofstra University on September 26, 2016 in Hempstead, New York. The first of four debates for the 2016 Election, three Presidential and one Vice Presidential, is moderated by NBC's Lester Holt.
Now playing
01:57
The first presidential debate in under 2 minutes
NEW YORK, NY - JANUARY 28:  Miss Universe, Venezuela's Alicia Machado, kisses Donald Trump, owner of the Miss Universe pageant, 28 January during her daily fitness workout at a health center in New York. The 19 year-old Machado started a new fitness program after she was criticized for gaining weight after winning the 1996 Miss Universe pageant. She claims to have lost five pounds in three days. AFP PHOTO Jon LEVY  (Photo credit should read JON LEVY/AFP/Getty Images)
JON LEVY/AFP/Getty Images
NEW YORK, NY - JANUARY 28: Miss Universe, Venezuela's Alicia Machado, kisses Donald Trump, owner of the Miss Universe pageant, 28 January during her daily fitness workout at a health center in New York. The 19 year-old Machado started a new fitness program after she was criticized for gaining weight after winning the 1996 Miss Universe pageant. She claims to have lost five pounds in three days. AFP PHOTO Jon LEVY (Photo credit should read JON LEVY/AFP/Getty Images)
Now playing
01:38
Clinton blasts Trump over treatment of beauty queen
AFP/Getty Images
Now playing
02:15
Trump repeatedly interrupts Clinton
clinton trump debate reality check stop frisk foreman_00002004.jpg
clinton trump debate reality check stop frisk foreman_00002004.jpg
Now playing
01:29
Debate reality check: Does 'stop and frisk' stop crime?
clinton trump debate hofstra temperament bts_00004205.jpg
POOL
clinton trump debate hofstra temperament bts_00004205.jpg
Now playing
01:15
Trump and Clinton battle over presidential temperament
donald trump bill clinton debate sot_00000724.jpg
donald trump bill clinton debate sot_00000724.jpg
Now playing
00:45
Trump: Glad I held back on Bill Clinton's indiscretions
Now playing
00:52
Hillary Clinton: Donald Trump says crazy things
Donald Trump speaking at the 1st Presidential Debate at Hofstra University, New York on September 26, 2016
CNN
Donald Trump speaking at the 1st Presidential Debate at Hofstra University, New York on September 26, 2016
Now playing
02:06
Trump: I was against Iraq War, ask Sean Hannity
Getty Images
Now playing
01:23
They can't say that on a debate stage...can they?
The First Presidential Debate
The First Presidential Debate
Now playing
03:08
Welcome to Hofstra-Palooza
Master of Ceremonies, Lester Holt speaks at the 31th Annual Great Sports Legends Dinner to benefit The Buoniconti Fund to Cure Paralysis at The Waldorf Astoria Hotel on September 12, 2016 in New York City.  (
Bryan Bedder/Getty Images for The Buoniconti Fund
Master of Ceremonies, Lester Holt speaks at the 31th Annual Great Sports Legends Dinner to benefit The Buoniconti Fund to Cure Paralysis at The Waldorf Astoria Hotel on September 12, 2016 in New York City. (
Now playing
03:03
Lester Holt in spotlight as moderator of first debate
do debates matter john king magic wall origwx js_00005406.jpg
do debates matter john king magic wall origwx js_00005406.jpg
Now playing
03:04
Do debates affect a presidential race? Sometimes.
that one debate moment origwx GR_00023324.jpg
that one debate moment origwx GR_00023324.jpg
Now playing
03:44
Inside two of the toughest debate moments ever
JFK Nixon heavyweight orig_00000322.jpg
JFK Nixon heavyweight orig_00000322.jpg
Now playing
00:54
The great debate: Kennedy vs. Nixon

Story highlights

Zingers can fade after the moment, but gaffes can linger

Viewers will be on alert for any stumbles that might seal the fate of Clinton or Trump

Watch the first presidential debate tonight at 9 p.m. ET on CNN, CNNgo and CNN.com.

(CNN) —  

Presidential debates are about ideas, tough questions – and gaffes.

The impact of one line zingers can fade after the moment, but gaffes can linger, altering a race and shifting the momentum.

Mistakes happen in almost every debate. And as Hillary Clinton and Donald Trump meet for their first clash Monday – 56 years to the date after the famous showdown between Richard Nixon and John F. Kennedy – viewers will be on alert for any stumbles that might seal the fate of either candidate.

Here are eight of the biggest blunders from nearly six decades of presidential debates:

Nixon vs. Kennedy, 1960

The first televised general election debate featured a candidate who understood the power of television and one who didn’t. John F. Kennedy, tall, young and confident, looked into the camera and addressed 80 million viewers.

Richard Nixon, pale and recovering from the flu, shifted his eyes, looking unfocused as he addressed the questioners rather than the viewers. Nixon was also sweaty and unshaven. People who listened to the debate on the radio thought Nixon won. People who watched the debate thought Kennedy won. The next day’s Chicago Daily News headline summed up the importance of the visuals: “Was Nixon Sabotaged by TV Makeup Artists?”

Donald Trump’s 6 debate tactics

It went so badly for Nixon that for the next 16 years, general election candidates avoided televised debates.

Ford vs. Carter, 1976

So memorable was President Gerald Ford’s gaffe that it made it into his obituary. Asked about the expansion of Russian power in Eastern Europe, Ford flatly denied it.

Carter and President Gerald Ford debate domestic policy at the Walnut Street Theater in Philadelphia on September 23, 1976. It was the first of three Ford-Carter presidential debates.
White House Photo/National Archives
Carter and President Gerald Ford debate domestic policy at the Walnut Street Theater in Philadelphia on September 23, 1976. It was the first of three Ford-Carter presidential debates.

“There is no Soviet domination of Eastern Europe, and there never will be under a Ford administration,” he said.

Flummoxed, the moderator gave Ford a chance to modify his response. But Ford stuck by his response throughout the debate, insisting that Poland, Yugoslavia, and Romania were all “independent and autonomous.”

Sometimes thought of as “the accidental president,” Ford’s bumbling response only highlighted the sense among some that he wasn’t quite right for the job.

Dukakis vs. Bush, 1988

The question wasn’t hard.

“Governor,” CNN’s Bernard Shaw asked, “if Kitty Dukakis were raped and murdered, would you favor an irrevoc