“We’re in the end game,” said Jeff Link, a longtime Iowa-based Democratic strategist. “We’re just in the beginning of the end game, but we’re in it. This is when candidates feel the heat from others and they need to respond. This is when candidates will feel the need to make a move and distinguish themselves.”

The sniping escalated amid a New York Times/Siena College poll last week that showed Buttigieg gaining ground as Biden lost support in the state. The Times and Siena followed up with a batch of general election swing state polls showing Biden faring best against Trump when compared to Sanders and Warren.

Fueling the change in tone has been Biden’s slide and Warren’s steady rise from the basement of the polls to frontrunner in Iowa and New Hampshire. Her ascent has put her squarely in the sights of Biden and Sanders, who is competing more aggressively with the Massachusetts senator for progressive voters.

At the same time those three have started engaging more often with each other, the candidates polling below them have watched with envy Buttigieg's rise to second place in Iowa. His surge has made him a target for Harris, who announced last week that she is redeploying staffers to the state as part of a major campaign shakeup. The redoubling of her effort in Iowa amounts to an all-or-nothing gamble on her performance there.

Perhaps the biggest shift in rhetoric has come from Warren. The Massachusetts senator has run the model of a disciplined campaign, steering clear of attacks on her primary rivals and saving her barbs for President Donald Trump.

Iowa voters attend a campaign event with Sen. Elizabeth Warren in Dubuque, Iowa, on Saturday. | Scott Olson/Getty Images

Last week, she set aside her persona as happy Democratic warrior. She suggested that Biden was “running in the wrong presidential primary” after the former vice president attacked her Medicare for All financing plan. Biden’s campaign manager fired back on Twitter, noting that Biden is a lifelong Democrat, while Warren was a Republican until she was 47 years old.

Then, in remarks before some 13,500 attending the Iowa event, Warren poked at candidates running what she called consultant-driven campaigns based on fear.

“Anyone who comes on this stage and doesn’t understand that we are already in a fight is not the person who is going to win that fight,” Warren said. “Anyone who comes on this stage and tells you they can make change without a fight is not going to win that fight.”

The swipe appeared targeted at Biden’s consultant-heavy campaign, as well as at Buttigieg, who earlier in the night talked about not fighting for the sake of fighting.

“Pete’s been pretty clear that people go to Washington talking about fighting for us [and] then the focus is more on the fighting and less on the us,” said a campaign advisor to Buttigieg.

Is the crossfire a precursor to a heated debate Nov. 20? “It seems to be headed that way,” the adviser said. “Pete started making these contrasts even before the last debate and he’ll keep making policy contrasts with his competitors.”

Buttigieg also irked both Harris and Castro after saying the Democratic primary is really “getting to be a two-way” contest between him and Warren.

Harris called the comment “naïve,” leading Buttigieg to back away somewhat and say the race is still “fluid.” Castro played up the racial dynamics of the race.

“Anyone who thinks this is a two-person race doesn’t know anything about the black and Latino communities,” Castro said Sunday at the Warren County Democratic Party.

"Just look at his track record as mayor,” Castro told reporters. “He has a bad track record with African Americans on the issues.”

Buttigieg, the mayor of South Bend, Ind., responded Monday morning while on a bus tour through Iowa.

“I would be happy to walk him around South Bend and introduce him to folks if he wants to learn more about how we can tackle these really tough issues,” Buttigieg said.

Buttigieg has grown accustomed to sniping from rivals. Former Rep. Beto O’Rourke, who dropped out of the race last week, had spent the latter days of the campaign sniping at Buttigieg.

So far, Buttigieg hasn’t overtly gone after Biden, even though the elder statesman is widely perceived to appeal to the same type of moderate Rust Belt voters whom the mayor hopes to win over. Biden also hasn’t gone after Buttigieg, who recently eclipsed the former vice president in Iowa polls.

South Bend, Ind., Mayor Pete Buttigieg has grown accustomed to sniping from rivals. | Chip Somodevilla/Getty Images

Instead, Biden for nearly two months has attacked Warren and Sanders as an out of touch left-wing tandem. He singled out their advocacy for Medicare for All, which Biden first attacked in September as he started to nosedive in early state polls.

The Biden campaign had its most caustic exchange last week with Sanders when it accused the Vermont senator of not being honest about how Medicare for All would scrap Obamacare and lead to the loss of private health insurance and higher middle class taxes.

Sanders’ campaign struck back by saying overall out-of-pocket expenses would be lower for the middle class under his plan and accused Biden of “peddling dishonest insurance company talking points about 'Medicare-for-all.’”

But after Warren released her Medicare proposal, Sanders told ABC News her Medicare business tax “would probably have a very negative impact on creating those jobs, or providing wages, increased wages and benefits for those workers.” He called his own plan “far more … progressive, because it’ll not impact employers of low wage workers but hit significantly employers of upper income people.”

Warren denied the charge and insisted her plan wouldn’t cost employers more.

Scott Mulhauser, a political consultant with the Democratic firm Bully Pulpit Interactive and a former 2012 Obama-Biden campaign advisor, said the charges, counter charges, criticisms and contrasts are just starting.

“It’s right on time. It’s fall. Voting is approaching,” he said. “The field is narrowing — and the attacks are flying.”

But the attacks haven’t so far been so bad, according to New Hampshire Democratic Party Chair Ray Buckley.

“The elbows so far have been pretty gentle,” Buckley said. “So I don’t have any concern right now. I think the reason for that is everyone is committed to making sure Donald Trump is not in the White house in January 2021.”