(CNN) Democratic hopeful Pete Buttigieg signaled the start of a new political era for Sen. Bernie Sanders, one of the front-runners for the party's 2020 presidential nomination, with five crisp words.

With those five words during last week's CNN town hall marathon -- "No, I don't think so" -- the South Bend, Indiana, mayor briskly rejected Sanders' proposal to allow convicted felons to vote not only after, but during, their incarceration. In the process Buttigieg did something that Hillary Clinton almost never did during her 2016 primary contest against Sanders: He directly criticized the substance of one of the senator's policy proposals.

Buttigieg's dismissal of Sanders' idea to let felons vote -- and the criticism from the mayor and other candidates of the senator's call to eliminate private health insurance -- clearly indicated that the 2020 field isn't planning to reprise Clinton's approach of minimizing direct conflict with the Vermont independent.

"Whether it was the right strategy or the wrong strategy for Clinton, it was a very hands-off approach in which all of Bernie Sanders' idea were allowed to be untouched," said Jim Kessler, executive vice president for policy at Third Way, a centrist Democratic group often critical of Sanders. "And this is a very different time. For the first time really in his career, there is going to be scrutiny over his policy ideas."

Sanders' agenda, seen as the liberal vanguard in 2016, has clearly widened its support in the party since, a dynamic symbolized by the House Rules Committee hearing scheduled for Tuesday on the single-payer "Medicare for All" proposal he has championed since his 2016 race. A wide array of 2020 contenders -- including centrists such as former Colorado Gov. John Hickenlooper and former Vice President Joe Biden at his first rally Monday afternoon -- and Democratic House members have also embraced his proposal for a federal $15 minimum wage.

Liberal groups sympathetic to Sanders say he'll benefit from greater scrutiny from other candidates than he received in his last race. Neil Sroka, communications director for Democracy for America, says greater debate over Sanders' ideas will make it easier for him to identify himself as the most progressive alternative in the field.

"People having to declare where they are on things like Medicare for All, and the rights for people to vote while in prison, is a phenomenal sorting mechanism for this party," Sroka says.

But there's no question that greater scrutiny and pushback from the other candidates will expose Sanders to challenges he never truly faced from Clinton in 2016. Though his core idea of transforming the health care system into a single-payer structure has more support than in 2016, it remains far short of a majority in either chamber and has drawn al most no backing from congressional Democrats in swing states and House districts . (The $15 minimum wage, though much more likely to pass the House, for now also remains just short of a majority amid resistance from some swing district members.)

Deeply ideological, and often prickly when challenged in media interviews, Sanders has often brushed away criticism of his agenda by dismissing any skeptics as servants of "the billionaire class." In the coming months, he'll likely need more supple responses to fellow Democrats questioning ideas such as single-payer health care or tuition-free public college from a center-left perspective.

"He's being treated like a serious candidate, and his ideas are being treated like they are serious ideas," said Kessler.

A choice by Clinton

The absence of debate over Sanders' agenda in 2016 reflected a c onscious choice by front-runner Clinton's campaign . Despite Sanders' unexpectedly strong performance, her camp's confidence that she would ultimately win the nomination never wavered after she demonstrated a commanding advantage among African American voters in the South Carolina primary early in the calendar. Because Clinton always expected to clasp the nomination, she sought to minimize conflict with Sanders so as to limit rifts with his supporters that she would need to bridge in the general election. (As it turned out, bad blood with the Sanders camp persisted anyway and hurt Clinton that November.)

"The attacks in 2016, frankly, did not come (from Clinton) to the magnitude that could have been expected," said Tad Devine, a senior adviser to Sanders during that race. "There wasn't that level of engagement. I think their general strategy was: We're not going to engage."

One senior official in the 2016 Clinton campaign, who asked not to be identified in order to discuss its internal deliberations at the time, confirmed Devine's perspective.

"I think there were two decisions about how to take on Bernie Sanders in 2016," the former official said. "One was not to litigate ideological turf wars within the Democratic primary and two was to more often than not avoid being negative towards him at all. Both were driven by the assumption that she would be the nominee, and we wanted those voters who voted against her in the primary to have an easy transition to supporting her in the general."

To the extent that Clinton did criticize Sanders during the primaries, she mostly questioned not the desirability but only the political and legislative viability of his ideas. On Sanders' signature proposal to create a single-payer health care system in which the government would replace all private insurance, for instance, Clinton offered almost no assessment of whether it was a worthwhile goal. Instead she argued that single payer was unlikely to pass Congress and questioned whether it was wise to reconfigure the health care system again so soon after passing the Affordable Care Act in 2010.

"Rather than letting the Republicans repeal it or rather (than) starting all over again, trying to throw the country into another really contentious debate, let's make the Affordable Care Act work for everybody," she said at an April 2016 CNN debate with Sanders in Brooklyn.

Clinton often wrapped herself in the mantle of Barack Obama, portraying her ideas as an extension of his and Sanders' agenda as an implicit rejection of them. The sole issue on which Clinton consistently criticized Sanders during the primaries was one where she could position herself to his left: gun control, a question on which he had taken some positions sympathetic to the National Rifle Association while representing the heavily rural state of Vermont.

Apart from guns, says Devine, who is not advising Sanders again for 2020, "They never went after him as hard as they could have; instead, she presented her alternative view. There was never this big fight of: That's a bad idea. They didn't want to have a big debate on these issues."

This time many party centrists are spoiling for a fight over some key elements of Sanders' agenda. Buttigieg, Beto O'Rourke and Amy Klobuchar have already renounced Sanders' idea of eliminating all private health insurance (except for a few supplemental services) in his single-payer plan as too disruptive to the 180 million Americans who receive their coverage through that means; Biden, who joined the race last week, signaled in his remarks Monday that he would join them.

Who does more engagement help?

Sroka, the Democracy for America aide, says Sanders will benefit from more engagement than in 2016 over whether to maintain private insurance.

"The point is, I think, that discussion of what it means when you say, 'Health care is a right,' vs. Joe Biden saying, 'You have a right to buy health insurance,' is a good discussion," he says. "I frankly think most people when they hear it are going to side with the progressive case."

But Kessler of Third Way says a more forthright debate over single payer will cause many Democratic voters to question whether they want to centralize virtually complete control over the delivery of health care in the federal government. After Trump and the Republican-led Congress passed a tax bill in 2017 that cut taxes for red states in part by limiting the state and local tax deduction that mostly benefits blue states, Kessler notes, it's easy to imagine a future Republican president manipulating a single payer plan to slash reimbursement rates for hospitals and doctors in blue coastal states, which generally have higher costs than red interior states. If the federal government controlled all health care decisions, he notes, a future conservative president also would inevitably seek to limit services in culturally sensitive areas.

"If you have a government-run single payer health care, contraception is going to be a fight; abortion, too," Kessler says. "Anything having to do with LGBT rights is going to be a battle. What's going to happen with immigrants, noncitizens, people who are here illegally?"

But Devine said focus groups conducted for the Sanders campaign in Iowa last time found even many Democratic voters hesitant about completely eliminating tuition and instead drawn to the notion that students and their families should contribute some of the cost.

"There was a lot of pushback from that," said Devine. "You would sit in the group and people would talk about the kid down the street who's a slacker, do you want to give free tuition?" Several 2020 contenders have already questioned the idea, Sen. Amy Klobuchar of Minnesota most prominently.

Free public university tuition for families into the upper middle class also faces criticism from another angle: groups pushing to increase admissions of low-income and minority students to high-performing public colleges and universities. They worry that those students will be displaced if more middle- and upper-middle-class white families choose free public universities over expensive mid-tier private schools.

The lack of a sharp issue debate in 2016 may be one reason ideology was only a modest fault line in the Clinton-Sanders race: The senator ran somewhat better among liberal than among moderate voters, but the difference wasn't nearly as significant as other divides, particularly age and partisanship (with Sanders relying more on independents than on partisan Democrats), according to a cumulative analysis of the 2016 exit polls conducted by CNN . More focus on the implications -- and the formidable cumulative cost -- of Sanders' agenda could sharpen the ideological fault lines in the race, particularly with Biden, the other candidate leading in early polls, already likely to draw mostly from older and more moderate voters.

Even with the other contenders likely to challenge Sanders more aggressively than Clinton did, polls show a substantial audience within the Democratic coalition for many of the key ideas he has advanced. That means there are no guarantees that his critics will win a more forthright debate than Clinton instigated over them in 2016. But it seems guaranteed Sanders will face such a debate this time -- and that means his new wave of rivals will test him in ways Clinton never did.