Hillary Clinton and Donald Trump surged on Sunday towards another round of pivotal presidential primaries as their party leaders faced new questions about internal divisions that could complicate their nominees' general election chances.
Republican front-runner Donald Trump looked ahead to Tuesday's contests in five north eastern states where he is poised to do well and to a foreign policy speech later in the week.
His main rival, Texas Senator Ted Cruz, meanwhile, abandoned the Tuesday states and instead campaigned in Indiana, which votes on May 3.
On the Democratic side, underdog Bernie Sanders, meanwhile, faced thousands of supporters in Rhode Island, looking to the smallest state in the US for momentum even as he appeared to soften his attacks on Mrs Clinton.
"If Secretary Clinton is the nominee - and we're not giving this thing up, we're going all the way to California - but if she is the nominee, I would hope that she puts together the strongest progressive agenda," Mr Sanders said on ABC TV's This Week before courting voters in Rhode Island's capital city of Providence.
Mrs Clinton hoped Tuesday's contests in Rhode Island, Pennsylvania, Connecticut, Maryland and Delaware would mark a turning point in her quest for the Democratic nomination. Victories in four or five states would all but cripple Mr Sanders's White House bid.
The former secretary of state opened her day at a Philadelphia black church ahead of the primary in Pennsylvania, Tuesday's top delegate prize.
"If you will vote for me, I will stand up for you throughout this campaign. I will continue to stand up and fight all the way into the White House," Mrs Clinton told parishioners at Triumph Baptist Church in North Philadelphia.
Mrs Clinton, emerging stronger after a triumph in last week's New York primary, stood to effectively lock up the nomination on Tuesday.
The five states together offer 384 delegates, and Mrs Clinton was expected to win most of them, putting her closer to gaining the trove that would put her closer to security the 2,238 delegates needed to secure the nomination at the Democratic national convention in July.
With 172 delegates at stake Tuesday on the Republican side, Mr Trump could take a significant step toward his party's delegate majority with the dominant performance that many polls predict.
His rivals, Mr Cruz and Ohio governor John Kasich, have been mathematically eliminated from earning the necessary 1,237 delegates and are instead trying to block Mr Trump from the majority to force a contested national convention in July in which delegates would be free to turn from Mr Trump to other candidates after the first ballot.
Mr Trump was set to campaign in Maryland as a senior adviser charged that Mr Cruz is "going to lose all five states and probably finish third in most of them" on Tuesday.
The adviser, Paul Manafort, said the billionaire businessman's campaign, not the candidate, was evolving as the general election neared, an attempt to clarify his recent comments to the Republican National Committee that Mr Trump has been "playing a part" onstage and would soon start to display "more depth ... the real person," in new settings.
That includes working with such Washington stalwarts as Senate Majority Leader Mitch McConnell - even as Mr Trump casts himself as the ultimate outsider.
"What we're trying to do right now is work with the Mitch McConnells" on party business, Mr Manafort said on Fox News Sunday.
"We have to work with these people," he said. "What I was tasked to do this past week, including going to the RNC meeting, was to (convey) that the campaign cares about them and we will run some traditional elections."
Meanwhile, both parties acknowledge deep intra-party divisions as the general election season approaches.
"It's pretty split," RNC chairman Reince Priebus said of his party, noting that he is aware that some Republicans are calling for a third-party bid to challenge Trump in the general election.
But Mr Priebus said such a challenge was "a nothing burger", any third-party bid or write-in campaign could doom the Republicans in November.
Democratic National Committee chair Debbie Wasserman Schultz downplayed tensions between Mr Sanders and Mrs Clinton, whose rivalry has become increasingly nasty in recent weeks.
"Regardless of the intensity of what's played out here...we are going to be unified," she declared.