After months of talk about the potential of a contested Republican convention, Trump, the presumptive GOP nominee, is quickly consolidating his party's support -- something Clinton is unable to do with Sanders still in the race.
With only a few major nominating contests left, including California and New Jersey on June 7, Sanders lacks a credible mathematical path to overhauling Clinton's wide lead in pledged delegates. And with polls showing Clinton's general election advantage over Trump evaporating, a lingering fracture in the Democratic party could be perilous for its chances to keep the White House.
Still, Sanders is not heeding calls from some Democrats to get out of the race -- or at least cool his rhetoric during the final weeks of the primary season. Instead, he kept up his blistering criticism of Clinton over the weekend and deepened his feud with the party establishment, including endorsing the primary challenger to Democratic National Committee Chairwoman Debbie Wasserman Schultz.
"The last I heard is that we are a democratic country, and that elections are about vigorous debates over the issues. Secretary Clinton and I disagree," Sanders told Jake Tapper Sunday on CNN's "State of the Union." "What the Democratic leadership has got to understand is that not all of my supporters go to these fancy fundraising dinners. They're working people who are hurting now, who want real change in the economy."
He added: "I hope the Democratic leadership understands they have to open up the process, bring those people in."
Sanders acknowledged in the interview that he has a "very, very uphill fight" in his quest to overtake Clinton, given that he has won 46% of pledged delegates so far and she has won 54%. But he rebuked Democratic superdelegates -- party office holders and lawmakers who can vote however they choose at the convention -- for overwhelmingly coming out for Clinton early on in what he said was an "anointment" by the establishment and big money interests.
Clinton's failure to finally put away the Sanders campaign is grating on the former secretary of state.
"I will be the nominee for my party,'' Clinton told CNN's Chris Cuomo in an interview last week. "That is already done, in effect. There is no way I won't be.''
On Sunday, she said there will be an "obvious need of us to unify the party" once she becomes the presumptive nominee.
"I will certainly do my part, reaching out to Sen. Sanders, reaching out to his supporters," she said on NBC's "Meet the Press." "And I expect him to do his."
The internal conflict comes at a time when polls show that Trump is getting a dividend from closing out the Republican primary race, and setting up what could be a close election against Clinton in November.
A CNN/ORC poll from the beginning of May
found Clinton leading Trump by 13 points. But in more recent surveys, there is increasing evidence of a bump in polling as Trump consolidates GOP support.
A Fox News poll last week showed Trump leading Clinton 45% to 42%, findings that were within the survey's margin of error. Meanwhile, a New York Times/CBS News national survey released Thursday had Clinton up by six points.
A Washington Post/ABC News poll Sunday
found a statistical dead heat, with Trump leading Clinton 46% to 44% among registered voters, figures that represent an 11-point swing since March. The former secretary of state leads by six points among all adults -- down from an 18-point lead in March.
Quinnipiac University polls in swing states Ohio, Florida and Pennsylvania this month also had the rivals effectively neck-and-neck.
Polls this far out from a general election cannot offer a reliable picture of what will happen in November. But they can shape the political environment in which the early stages of the race evolves and, if they continue to show Trump gaining in strength, are likely to increase pressure on Sanders to bring the Democratic Party together.
But Sanders does not see such polls as an argument that he should get out of the race or dial back attacks on Clinton. In fact, he sees them as proof that he would be a superior general election candidate to the former first lady -- most polls show him leading Trump.
'I have been vetted'
In the NBC interview, Clinton suggested Sanders simply hasn't been subjected to the rough and tumble of politics the way she has.
It's "fair to say that I have been vetted and tested, and I think that that puts me in a very strong position," she said.
Referring to Sanders, she said, "let me say that I don't think he's had a single negative ad ever run against him."
Sanders disputed the notion that he is only doing better than Clinton because he has not had to endure the years of partisan warfare that have shredded her approval ratings.
"Any objective assessment of our campaign versus Clinton's campaign, I think, will conclude we have the energy, we have the excitement, we have the young people, we have the working people, we can drive a large voter turnout, so that we not only win the White House, but we retain, regain control of the Senate, do well in the House and in governor's chairs up and down the line," Sanders told Tapper.
Latest polls clearly show that the lingering Democratic Party divisions are a challenge for Clinton.
The Washington Post/ABC poll released Sunday showed that in a match-up equation with Trump, Clinton currently gets 86% of Democratic voters, meaning that a slice of the party coalition that is not yet sold on her as nominee.
Making a decision to leave a primary race or to tone down attacks on a rival who appears headed for victory is the toughest choices any candidate faces. It is a particularly acute dilemma for Sanders, given that he has won millions of votes, ignited a populist uprising in the Democratic Party that no one saw coming and is basking in an unprecedented reception to his democratic socialist ideas that left him in the political wilderness for years in the Senate.
He and his campaign team have dismissed the idea that he could wreak lasting damage on Clinton if she becomes the nominee, saying he will do whatever it takes to ensure that Trump does not win the presidency. But if his arguments about the process of the Democratic primary race leave the impression that he has been unfairly treated and that Clinton is somehow not the legitimate nominee, the task of uniting the party becomes far more difficult.
Sanders vs. DNC
That's where Sanders' clash with Wasserman Schultz is particularly concerning to some Democrats. The Vermont senator's campaign has consistently accused the DNC chairwoman of tilting the race in favor of Clinton and criticized the scheduling of debates on Saturday nights when television audiences are lower, and the closed primaries that bar independents in big states like New York.
Sanders is now backing Wasserman Schultz's primary opponent in her Florida district, Tim Canova, and left no doubt about the strengths of his feelings about her on Sunday.
"Well, clearly, I favor her opponent," Sanders told Tapper. "His views are much closer to mine than as to Wasserman Schultz's." "In all due respect to the current chairperson. If (I was) elected president, she would not be reappointed to be chair of the DNC."
Sanders campaign manager Jeff Weaver also sent out a fundraising email to supporters Sunday seeking contributions for Canova.
For all the sudden handwringing in Democratic circles, it's still likely that Clinton will enjoy her own boost in the polls once she finally becomes the presumptive Democratic nominee similar to the one Trump is enjoying now.
For now, veteran Democrats appear to be ready to give Sanders some room. But the clock is ticking.
"After (Clinton) has actually won, after she actually has enough delegates to win the nomination, I think Bernie needs to think about his legacy," said Mark Alderman, a top Democratic Party donor who was part of President Barack Obama's transition team. "Bernie is in the middle of the tsunami -- he doesn't have any perspective yet. It will take a little time. Unfortunately, he only has a little time. He has got to get it together by July."