(CNN) Beto O'Rourke sharply criticized Pete Buttigieg this week after the South Bend, Indiana, mayor called O'Rourke's push for mandatory buy-backs of assault-style rifles a "shiny object."

The former Texas congressman and 2020 Democratic presidential candidate characterized Buttigieg's position as driven by political calculation.

"I was really offended by those comments," O'Rourke told reporters after a gun control policy forum in Las Vegas on Wednesday. "And I think he represents a kind of politics that is focused on poll-testing and focus-group-driving and triangulating and listening to consultants, before you arrive at a position."

O'Rourke's comments came after Buttigieg took an implicit swipe at O'Rourke -- as well as New Jersey Sen. Cory Booker, who was the first 2020 presidential candidate to support mandatory buy-backs -- on Wednesday.

He called mandatory buy-backs "a shiny object makes it harder for us to focus" on reforms that are more politically viable in Washington, where any new gun control measures would require Republican support given the GOP's Senate majority.

"As a policy, it's had mixed results," Buttigieg said. "It's a healthy debate to have, but we've got to do something now."

Mandatory buybacks are a dividing issue for the 2020 Democratic field at large. But O'Rourke's direct and personal rebuke of Buttigieg forced into the open a tension that has brewed for months between the two camps and their supporters. The two entered the race in the same lane: Both as generation-change candidates with viral appeal, both vying to be seen as alternatives to former Vice President Joe Biden.

O'Rourke's allies have been bitter about Buttigieg since early April, when he said in New Hampshire -- in a clear poke at O'Rourke -- that "I hear the way you ingratiate yourself to voters is to stand on things."

Both have lost altitude in recent months, though Buttigieg has remained a much stronger fundraiser and is ahead of O'Rourke in nearly all polls.

But the divide over mandatory buy-backs has offered the clearest view yet of their diverging approaches to politics -- with Buttigieg seeking to show he is able to stake out a sober middle ground and could function effectively in Washington, and O'Rourke relying on more emotional appeals to those wishing to shake up the political system.

Ongoing clash

Buttigieg often uses the term "shiny object" to describe something he views as a distraction. Shortly after his appearance at Wednesday's forum, Buttigieg was asked whether he would allow his hypothetical vice president's son or daughter to serve on a foreign board.

He responded by saying Trump is trying to "change the subject" away from his "abuse of power."

"Let's deal with that and not get caught in the shiny objects he's going to throw out," Buttigieg said.

But the mayor calling O'Rourke's signature gun proposal a "shiny object" set off the Texas Democrat and his campaign.

The two had already clashed in September , when Buttigieg said O'Rourke's push for mandatory buy-backs -- which he has called for in the wake of a mass shooting in his hometown of El Paso, Texas -- played into the hands of Republicans looking for reasons to back away from gun control measures at a time when Trump and Senate Majority Leader Mitch McConnell were "at least pretending to be open to reforms."

But the Las Vegas forum marked a clear escalation from O'Rourke.

After pointing to Buttigieg's comments, O'Rourke asked, "How in the world can you say that to March For Our Lives," referring to the student-led group founded in the wake of last year's school shooting in Parkland, Florida, that co-hosted Wednesday's forum.

"How can you say that to survivors of mass shootings across this country? How can you say that to the majority of Hispanics in America, certainly in Texas, who fear that they will be the victims of a mass shooting inspired by racism, hatred, that's been welcomed into the open by this president, and has been armed with weapons of war?" he said.

O'Rourke lashed out again in a text message conversation with BuzzFeed , saying that Buttigieg's comment was "offensive to anyone who has been shot by one of these weapons or who fears being shot or has lost someone to an AR-15 or AK-47."

For his part, Buttigieg on Wednesday laid out a more modest set of proposals broadly backed by the field of Democratic 2020 presidential contenders: closing loopholes that allow some guns to be sold without background checks; allowing states to adopt "red flag" laws that allow families or police to ask a judge to have a potentially dangerous person's guns taken away; and banning the sale of additional assault-style rifles.

"We can get background checks done now. We can get red flag laws done now. We can get something done about the new sale of assault weapons now. We cannot wait for these other debates to play out -- even if they're healthy debates -- to get that action done because lives are on the line," Buttigieg said.

Vying for the top tier

In the Democratic primary field, Booker was the first to call for mandatory buy-backs. O'Rourke announced his support in early August -- after the El Paso shooting -- and California Sen. Kamala Harris called it a "good idea" in early September.

However, Buttigieg and former Vice President Joe Biden have opposed mandatory buy-backs. Biden instead this week proposed that owners of assault-style rifles be required to participate in an optional buy-back or register them with the government under the same law that strictly regulated machine guns in the wake of the gangland shootings of the early 1920s and '30s.

But what's striking about the O'Rourke-Buttigieg clash is that both are coming from a similar starting point: They are vying to vault into the top tier of a Democratic presidential race currently being dominated by Massachusetts Sen. Elizabeth Warren, Biden and Vermont Sen. Bernie Sanders.

Buttigieg has positioned himself as a generational-change candidate with Midwest roots -- but also as a more moderate alternative to the party's most progressive candidates.

O'Rourke had offered similar appeal, with a Sun Belt bent. But his campaign changed direction in the wake of the El Paso shooting.

Previously O'Rourke had opposed mandatory buy-backs, saying as a Senate candidate in 2018 that he would not support taking away Americans' guns.

"To be clear, they should have them. If you purchased that AR-15, you own it, keep it. Continue to use it responsibly," O'Rourke said during that race.

But he said the shooting in his hometown had changed him.

"I know that this is not politically easy. It's frankly why far too few people have proposed it; it's frankly why I have not proposed it in the past," O'Rourke told CNN in El Paso in August, explaining his reversal. "I've said, 'This is something we should consider, I want to think about it, I want to talk to people about it.' I've thought about it. I've talked to people. ... And now, beyond a shadow of a doubt, regardless of what it does to our prospects going forward, you've got to speak the truth and be clear about where the solutions are."

He made headlines in the September Democratic presidential debate when he said that "hell yes," he would take AR-15s and AK-47s away from their owners.

That drew a rebuke from Booker, who already supported mandatory buy-backs -- as well as a national gun licensing program. Booker again criticized O'Rourke on Wednesday in Las Vegas.

"He saw the horrors visiting his community," Booker said of O'Rourke's reversal on the issue. "Are we going to have to wait until hell's lottery comes to your community? No, we are a better country."

O'Rourke responded on stage that he wanted to give Booker "all the credit in the world for being a leader on this."

"So Cory's deal is, you know, he's trying to determine who got to which policy position first," O'Rourke told reporters afterward. "My response to that is, we're both in the right place on this and so let's work together."