Miami (CNN) Two presidents, past and present, dominated the campaign trail Friday, as Barack Obama assailed Donald Trump for "lying" and "constant fear-mongering," prompting Trump to snap back that Obama had attracted a "very small crowd."

Obama campaigned in Florida and Georgia for Democratic candidates in two of the key races in Tuesday's midterm elections. In some of his sharpest criticism of Trump yet, the former President cast his successor as a compulsive liar with an authoritarian streak.

He hammered President Donald Trump and the GOP for claiming to support the Affordable Care Act's coverage protections for those with pre-existing conditions -- even as Trump's Justice Department is involved in a lawsuit to end those safeguards and the GOP congressional majority attempted to repeal the law.

"What we have not seen is politicians just blatantly, repeatedly, baldly, shamelessly lying -- just making stuff up," Obama said in Miami. "That's what they're doing right now, all the time."

Obama also accused Trump of "constant fear-mongering" and a disdain for the truth that was corroding the infrastructure of American political life.

"When words don't mean anything, when truth doesn't matter, when people can just lie with abandon, democracy can't work," Obama said.

Shortly afterward, Trump said in West Virginia that he'd watched Obama's speech on television aboard Air Force One.

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He criticized Obama over his false promise that under his health care law, people would be able to keep their doctors, and rejected his predecessor's advice that he cease attacking the news media.

"Lie after lie, broken promise after broken promise," Trump said of Obama. "That's what he did."

The former and current presidents are barnstorming the country ahead of the elections, in which Democrats hope to win control of the House of Representatives, gain Senate seats and pick up governor's mansions and state legislatures across the country ahead of the 2020 redistricting process.

As much as anything Obama and Trump said, the pair's travel schedules underlined a common purpose: turning out their parties' bases.

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Obama was in Miami and Atlanta, each home to minority voters that Democratic gubernatorial candidates Andrew Gillum in Florida and Stacey Abrams in Georgia need to turn out in force on Tuesday.

But the main thrust of his argument was more expansive, touching on the political left's existential concerns over what they see as hypocrisy in Trump and disdain for democratic norms. Obama scorned Trump and other Republicans for "vilifying others, questioning their patriotism" and then expressing concerns about civility.

In response to Trump's recent suggestions that he could revoke the promise of birthright citizenship with the stroke of a pen, Obama said that "a president doesn't get to decide on his own who's an American citizen and who's not," a rejoinder that delighted the partisans in Miami's Ice Palace Film Studios.

Obama's pitch was repeatedly interrupted by hecklers, leading him to ask at one point: "Why is it that the folks that won the last election are so mad all the time?"

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In line outside the venue, hours before Obama would take the stage, supporters -- who had followed Trump's recent trip to the state -- said they welcomed the juxtaposition.

"You have a president that's going to be here today and you have an entertainer that's in office right now," said Rashawn Watson, a Democrat from Miami Gardens. "Trump's entertaining base versus actually caring about America."

Obama argued much the same during portions of his speech, calling Trump's recent decision to send thousands of troops to the border to defend against a group of migrants that is still hundreds of miles away "a political stunt" -- and an affront to service members who "deserve better than that."

In West Virginia, meanwhile, Trump acknowledged that a message focused on the economy -- which congressional Republicans urged him for months to put at the forefront of the GOP's campaign efforts -- is sometimes "not as exciting to talk about ... because we have a lot of other things to talk about."

Trump defended a racist web video he had tweeted Wednesday depicting Democrats as responsible for an undocumented immigrant killing two police officers. ( A CNN review of the immigrant's case shows multiple interactions with local and federal officials in both Republican and Democratic administrations, as well as with noted anti-immigration firebrand Joe Arpaio, the former Maricopa County, Arizona, sheriff who received Trump's first presidential pardon last year.)

"They gave me a hard time because I put it up on Twitter," Trump told a crowd in Huntington. "They said, 'You shouldn't be doing that! That's not nice!' "

"And I say, all I'm doing is just telling the truth, what can I say?" he said.

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Trump also acknowledged that Democrats could win a congressional majority -- though he suggested he wouldn't be to blame if they do.

"I'm not saying that they won't squeak it by, maybe, because they've got a lot of races, and I can't go everywhere," he said.

The races in Georgia and Florida are, perhaps more than any other on the wide and varied 2018 map, a referendum on the first two years of the Trump era -- and whether Democrats have nailed down a path for defeating the President's acolytes in the Republican Party.

Obama won twice in Florida and lost by relatively narrow margins in Georgia (about 5 points in 2008 and 8 points in 2012) with pitches that largely predicted Gillum and Abrams, who have steeped themselves in progressive populism while being careful not to alienate moderate business interests. Like Obama, they have been mostly successful in synthesizing those messages with the aid of compelling personal narratives.

Gillum, the mayor of Tallahassee, could become Florida's first African-American governor with a victory next week. The election of Abrams, a former Georgia House minority leader, would make her the first female African-American governor in the country.

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Republican candidates in 2018 have, quite obviously, gone in a different direction.

Former Rep. Ron DeSantis of Florida, Gillum's opponent, and Georgia Secretary of State Brian Kemp, who's running against Abrams, won their primaries by latching on to Trump and all that comes with him. While both have made occasional nods to their party's moderate establishment, a necessity in Florida, where a rolling series of environmental crises -- from toxic algae bloom to wildfires and stronger storms -- are dominating the political atmosphere in both typically Republican and Democratic strongholds, neither has made any real attempt to persuade centrist Democrats.

Still, Obama had a heavier lift on his hands in Miami than Trump did in Fort Myers, where it seemed like just about everyone at his rally had already voted the party line.

"Who voted?" Trump asked the packed arena. When his question was met with an immediate roar, the President seemed genuinely taken aback. "Everybody voted already? No kidding. Let me see it again. Who voted?"

Again, the audience howled, leading Trump to deadpan: "Then what the hell am I doing here tonight?"