That momentum is vitally important, and the way analysts keep up with that momentum is through polls. We see hundreds of polls during a presidential race. They're valuable tools, but what they say doesn't always tell the whole story.
"The polls are misleading a little bit," said Frank Gilliam, dean of UCLA School of Public Affairs. "They say they're polling likely voters, but they're not polling the people who really matter, and those are the people who don't really pay a lot of attention to the election, but who will in fact vote."
Gilliam said it's not so much that the polls are wrong as much as that the people surveyed are not yet ready to make a decision.
"The question is come mid-September through mid-October when people really start paying attention, now what's going to happen?" he said.
When it comes to the presidential race between President Barack Obama and former Massachusetts Gov. Mitt Romney, experts say it's going to be a close one.
"I don't think there's any chance that this election is a landslide," said UCLA Professor Lynn Vavreck.
"One of the most remarkable things about this year is how remarkably stable the polls have been," said ABC News Anchor George Stephanopoulos, "showing basically it's been a dead heat for the last several months. I think it's an accurate reflection of where the race is."
When it comes to convention time, we often see a candidate suddenly surge ahead post-convention.
But in 1998, Democratic presidential candidate Michael Dukakis emerged from his convention with a seemingly insurmountable lead, 17-points over his rival then-Vice-President George H. W. Bush.
In the final two months before the election, Dukakis made some unexpected missteps and his political record was attacked.
Veteran ABC News journalist Sam Donaldson said Dukakis simply didn't fight back.
"Listen, the ruling politics is if you attack me in the morning, I answer that by noon and I hit you in the nose by the 5 o'clock news," he said. "You don't sit there."
On election night, Dukakis won only 10 of 50 states. It was a landslide victory for Bush.
Look for ongoing reports from David Ono at the Republican National Convention in Tampa, Fla. Eyewitness News will also be at the Democratic National Convention in Charlotte, N.C., in September.