Kennedy, who spoke in front of a Mustang on a car lift in the autoshop at Diman Regional Technical School in Fall River, Massachusetts, pitched the Democratic Party as a party that helps "Coal miners or single moms. Rural communities or inner cities. The coast or the heartland."
"So here is the answer Democrats offer tonight: we choose both," he said. "We fight for both. Because the strongest, richest, greatest nation in the world shouldn't leave any one behind."
Kennedy, who comes from one of the most prominent families in American politics, referenced helping immigrants, addressing those affected by the expiring Deferred Action for Childhood Arrivals program and at one point speaking to them in Spanish.
He indirectly referenced Trump as a "bully" during his response.
"Bullies may land a punch. They might leave a mark," the Massachusetts Democrat said. "But they have never, not once, in the history of our United States, managed to match the strength and spirit of a people united in defense of their future."
He also tied the turmoil of the past year to Trump's administration, saying, "We all feel the fault lines of a fractured country ... We hear the voices of Americans who feel forgotten and forsaken."
Kennedy slammed Trump for "targeting laws" that protect Americans.
"It would be easy to dismiss the past year as chaos. Partisanship. Politics. But it's far bigger than that," he said. "This administration isn't just targeting the laws that protect us -- they are targeting the very idea that we are all worthy of protection."
Early on during his speech, he explained, in his direct appeal to blue-collar workers in the US, why he chose a technical school as the location for his response.
"From textiles to robots, this is a place that knows how to make great things," he said. "The students with us this evening in the autoshop at Diman Regional Technical School carry on that rich legacy."
He continued: "It is a fitting place to gather as our nation reflects on the state of our union."
On Tuesday night, Trump's State of the Union address argued for bipartisan efforts
while trumpeting the roaring stock market and low unemployment. He struck a unifying tone after a year of stoking divisions on race, politics and gender.
"Tonight, I call upon all of us to set aside our differences, to seek out common ground, and to summon the unity we need to deliver for the people we were elected to serve," Trump said.