(CNN) Every leading Democratic presidential candidate now supports a renewed ban on assault-style weapons, an issue that t he party has considered too hot to touch for most of the past quarter-century.

But House Democrats, who are preparing for their first legislative hearing Wednesday on the issue in years, remain just short of the 218 votes needed to pass such a ban on the floor.

The contrast between the presidential candidates' aggressive embrace of a federal ban and the palpable caution of House Democratic leaders illuminates the enormously complex and challenging legislative landscape still confronting gun control advocates, even as polls now consistently show clear national majorities in support for the major components of their agenda , including a ban on assault-style weapons. The disparity is reminiscent of the gap between the fulsome rhetoric from leading Democratic presidential candidates about "Medicare for all" or a "Green New Deal" and the more modest support each measure has attracted in the House.

The bill to ban assault-style weapons remains short of a majority largely because of hesitance among just a few Democrats representing districts that voted for President Donald Trump in 2016, especially those with large blue-collar constituencies, according to a new CNN analysis of support for the legislation sponsored by Rhode Island Democratic Rep. David Cicilline.

While fully 95% of House Democrats in districts that Hillary Clinton carried in 2016 have endorsed Cicilline's bill, almost half of the Democrats in districts won by Trump in 2016 have not signed onto the legislation, the CNN analysis found.

"It's a tough issue for some people," Cicilline said in an interview on Monday. "I respect that people are making judgments based on what they think their constituents expect them to do, or they come to places where there are very different attitudes about firearms generally. We are a big caucus, a diverse caucus"

Few issues in the gun control debate generate more emotional on either side than the prospect of banning the assault-style weapons such as the AR-15 that have been repeatedly used in mass shootings. When Republicans won control of the House in 1994 for the first time in four decades, many Democrats attributed the defeat partly to the legislation they passed that year banning assault style weapons.

Gun control advocacy groups this year are prioritizing other bills -- particularly the "universal background check" legislation the House passed earlier this year and "red flag" laws to confiscate guns from owners who show signs of presenting a danger. But some advocates worry that it will signal weakness and embolden gun owner groups led by the National Rifle Association if the House cannot pass an assault ban while the Democratic presidential candidates are so loudly touting the idea-and in some cases even moving past it to propose, as former Rep. Beto O'Rourke has done , a mandatory buyback of the estimated 15 million assault-style weapons already in circulation.

"Despite the fact that assault weapons are only a small, but very visceral part of the gun violence mix, a lot of our base looks at that issue as very representative of gun control," said one leading gun control advocate who asked for anonymity to discuss the movement's internal political calculations. "And they are going to start asking questions about the ironclad commitment of Democrats to follow through on this if we don't move it."

The counterargument from other gun control advocates is that it doesn't make sense to push the wavering House Democrats to pass an assault ban this year knowing that Senate Majority Leader Mitch McConnell, who is already blocking a vote on the House's universal background check bill , will not allow it to come to a vote there. Some argue for delaying a vote until 2021 if Democrats win the White House next year so vulnerable House Democrats would have to vote only once for a ban on assault-style weapons.

"They don't want to make members in very marginal districts take a vote that's useless," says Matt Bennett, a veteran of pro-gun control groups who now serves as executive vice president for public affairs at the centrist Democratic group Third Way. "I think if there was hope for it in the Senate, Speaker Pelosi would force them to do it."

Caught between these competing considerations, Cicilline's bill HR 1296 remains stuck in a revealing legislative limbo. The bill has support from 208 Democratic co-sponsors and one Republican -- New York Rep. Peter King. Counting Cicilline and Speaker Nancy Pelosi, who in that role doesn't co-sponsor legislation but would surely back the bill if it came to the floor, that means the legislation has 211 votes, just seven short of what it needs to pass. Two other Republicans have expressed general support for a ban on assault-style weapons, though they have not put their name on the legislation.

Only 25 House Democrats in all, leaving aside Pelosi, have so far refused to endorse the bill -- but that's enough to keep it just short of a majority.

"We will continue to get support and continue to build toward that 218," Cicilline said in the interview. "I continue to think we will get there."

But Cicilline acknowledges "it's still up in the air" whether the bill will ever come to a floor before the 2020 election.

"Obviously it will be a decision made by the Speaker," he says. "I haven't spoken to her directly about it, and I won't do that until I have 218 votes. I need to be able to say with certainty that I have the votes before I ask the Speaker to bring the bill to the floor."

Both the level of support that Cicilline has attracted for his bill, and the difficulty he faces in crossing the last mile to a majority, testify to the evolving politics of gun control.

The support he's attracted from so many Democrats captures one key dynamic: Compared to the last time Congress seriously considered gun control legislation in the 1990s, Democrats are far more unified today in supporting such measures. During Bill Clinton's first two years, fully 69 House Democrats voted against the 1993 Brady Bill establishing the national background check system for purchases at gun stores , and 77 opposed the initial assault ban which Congress approved and Clinton signed in 1994. (The legislation had a ten-year sunset provision, and the Republican-controlled Congress allowed the ban to expire when it came due in 2004.)

The widespread opposition to both measures reflected a Democratic caucus that, at that point, still included dozens of members from rural, blue-collar and southern districts where the gun culture, and the NRA, were strong. (Both bills required strong crossover support from suburban Republicans to pass the House.) Even as late as 2009-2010, the last time Democrats controlled the House before this year, resistance from the remaining Democrats in such swing areas dissuaded the party from considering any new gun control measures.

But, as working-class white voters have become the foundation of the Republican coalition, Democrats lost many of their seats in such areas during the GOP congressional sweeps in the 2010 and 2014 mid-term elections. When Democrats rebuilt their House majority in 2018, they did so primarily by winning urban and white-collar suburban seats in metropolitan areas where support for gun control is strong.

That's allowed the party to achieve a level of consensus on gun issues unimaginable even a decade ago. Earlier this year, just two House Democrats opposed legislation the chamber passed imposing a "universal background check" requirement for all gun sales. Only seven voted against a measure to close the so-called "Charleston loophole" in the Brady Bill background check system.

House Democratic leaders and gun control advocates are confident they will also coalesce preponderant Democratic support for the package of gun control bills the House Judiciary Committee approved earlier this month. Those bills, expected to reach the floor later this fall, would provide grants to states to strengthen "red flag" laws (also known as "extreme risk protection orders') that allow courts to confiscate guns from individuals deemed dangerous; prevent individuals convicted of hate crimes from possessing firearms; and ban high capacity ammunition magazines that have been used in some mass shootings.

But the uncertain prospects of Cicilline's assault ban legislation suggests that the Democrats' new consensus on guns still faces stubborn legislative limits rooted in the geography of its House majority.

Though the House Democratic caucus is increasingly centered on metropolitan seats that are trending toward them, the last few districts that provide the party its majority include suburban seats and a handful of rural and blue-collar places that are at best evenly divided and more often lean slightly Republican in presidential contests. And while most members from such seats have been willing to accept the background check bills the party has advanced so far, the CNN analysis of support for Cicilline's bill shows that a ban on assault-style weapons remains a bridge too far for many of them.

That picture becomes clear through a series of revealing contrasts. The assault ban legislation has drawn overwhelming support from Democrats representing districts that voted for Clinton in 2016. Leaving aside Pelosi, 193 of the 203 Democrats in such Clinton districts have endorsed the bill, some 95%.

But Democrats in districts that voted for Trump last time are much more skittish. Of the 31 in such districts, just 16 have co-sponsored the bill, only about half. Although Democrats in districts that voted for Trump constitute about 13% of all House Democrats, the 15 of them not on Cicilline's bill constitute 60% of all the Democrats who haven't signed on.

Similar patterns emerge in the CNN analysis on other political measures. Fully 194 of the 208 House Democrats who won in 2018 by more than five percentage points (again excluding Pelosi) have endorsed the bill. Those who won by less than five points split much more closely: 15 are on the bill, 11 are not.

Demography is revealing too. Support for the ban is overwhelming in the Democrats' new sweet spot: urban and suburban districts with large numbers of college graduates and/or racial minorities.

Again, leaving aside Pelosi, just eight of the 134 Democrats in districts where the share of college graduates exceeds the national average are not sponsoring the bill. That means only six% of them have not endorsed it. In the 100 Democratic districts with fewer college graduates than average, 17 legislators have not sponsored the bill; that's nearly three-times as large a share.

Only 31 House Democrats still represent seats where the rural population exceeds one-fifth of the district's total. Eleven of them (more than one-third) have withheld support from the bill. Just 13 of the 194 Democrats (less than 7%) in seats with smaller rural populations have not endorsed the ban. (The rural share of the population isn't available for some districts.)

The epicenter of resistance to the ban is the point where these lines converge. Of the 15 Democrats in Trump districts who have not endorsed the bill, 10 won by five percentage points or less (and another two won by less than ten points.) Ten of the Trump-district Democratic hold outs also represent seats with fewer college graduates than the national average.

The ten Democratic hold outs from districts that Clinton won are an eclectic group. Only first-term Rep. Lizzie Fletcher in Houston among them won their own election by five points or less. But four of them -- Sharice Davids in Kansas, Peter DeFazio and Kurt Schrader in Oregon, as well as Fletcher -- hold seats that Clinton carried with well under 50% of the total vote. Perhaps most revealingly, seven of the ten hold seats with fewer college graduates than the national average. Four represent districts with relatively larger rural populations.

Cicilline says that in talking to colleagues resisting the bill, particularly those from Trump districts, the principal concern about the assault ban is that it triggers a trip wire in the gun control debate. With the exception of the ban on high-capacity ammunition magazines, the other gun control measures the Democratic House has considered so far regulate the kind of people who are allowed access to guns. But the assault-style ban limits the kind of gun that anyone is allowed to buy. "The biggest objection is unlike...most of the other gun safety legislation which is really focused on how to do you keep guns out of the hands of people who shouldn't have them...this prohibits the possession of a particular kind of weapon even by law-abiding citizens," Cicilline says.

The growing discussion of a mandatory buyback of the assault weapons already in circulation, now popularized on the campaign trail by O'Rourke, adds another complication to the Democratic calculations. Cicilline says such a provision "absolutely" would sink the bill banning future purchases of assault style weapons. But Rep. Eric Swalwell (D-CA), who touted the idea during his own now-ended bid for the Democratic presidential nomination, says he will seek a vote on such a mandatory buyback if the assault ban comes to the floor.

"My message to my colleagues is if you want to ban this weapon because it's dangerous for the future, you have to acknowledge it is dangerous now," Swalwell said in an interview. "Let's just do the buyback as well, recognizing that if [a voter] is opposed to the assault weapon ban, that voter is not going to be with us on anything. So, let's do what makes us the most safe."

Even without a vote on a buyback of assault-style weapons-which most observers consider unlikely to reach the floor-the struggle over regulating these weapons offers a telling reality check with implications far beyond the gun debate. The deeper message of the House hesitation on Cicilline's bill is that even if Democrats hold the House, or tip the Senate, many of the proposals their class of 2020 presidential candidates are promoting across a wide range of issues face a much narrower legislative path than the ringing applause on the campaign trail would suggest.