'TIME TO UNITE'

Her voice hoarse, Clinton said it was "time for us to unite as a country."

White House spokesman Josh Earnest criticized Trump's Republican rivals for declaring they would back him if he wins the party nomination for the November election.

"At some point, somebody in the Republican Party's going to have to step up and show some leadership," Earnest said.

Trump has emerged from the early contests with a clear lead in the delegates needed to capture the nomination at the party's July convention. US Senator Ted Cruz of Texas is second, with Rubio and Kasich trailing and counting on their home states to keep them in the race.

Trump's contentious campaign has been marked by harsh rhetoric against illegal Mexican immigrants and Muslims. There have been repeated clashes in recent days between his supporters and protesters, raising questions about whether the violence will hurt Trump in the primary race.

Rubio made a last-minute appeal to supporters at a rally at a college in West Palm Beach, Florida, speaking to a larger-than-normal crowd of 1,600, many of them students at the school.

He sounded alarmed at the prospect of Trump winning Florida and capturing the nomination.

"Leadership is not inciting people to get angrier," Rubio said. "That's not leadership. You know what it is? That's called demagoguery."

'PUNK ASS THUGGERY STUFF'

Former Alaska Governor Sarah Palin, who appeared at a Trump event in Tampa on Monday, said protesters were "trying to take away your rights" to gather peacefully.

"What we don't have time for is for all that petty, punk ass thuggery stuff that has been going on," she said, before she headed home to be with her husband, Todd, who was injured in a snowmobile accident.

Cruz held five rallies in the Chicago suburbs and told voters to support him if they wanted to stop Trump.

He said Trump had donated in the past to two Illinois Democrats - former Governor Rod Blagojevich and Chicago Mayor Rahm Emanuel - and asked voters to "remember who the moneybags is that's funding these politicians."

In the Democratic race, Clinton, a former secretary of state, hopes to pull away from Sanders, a US senator from Vermont, in Tuesday's voting. Polls gave her a big lead in Florida and North Carolina, but showed Sanders gaining ground in Ohio, Illinois and Missouri.

Sanders' win last week in Michigan, where polls indicated he trailed by double-digit margins, showed his ability to pull off a surprise.

He told a town hall sponsored by MSNBC airing on Monday that Trump was "literally inciting violence among his supporters" by offering to pay their legal fees.