Trump declares himself unbeatable if Republicans unite behind his candidacy
Donald Trump has called for Republicans to rally behind his presidential candidacy after he cruised to primary victories in three more states.
He declared that he could not be defeated in the November election as the standard-bearer of a united party.
On the Democrat side, Vermont senator Bernie Sanders handed former secretary of state Hillary Clinton a surprise loss in the industrial state of Michigan, increasing the likelihood that the contest for the party's nomination could stretch into early summer.
Mrs Clinton, however, crushed Mr Sanders in the southern state of Mississippi, continuing to win a large margin among black voters.
Speaking to MSNBC, Mr Trump said: "If the Republican party unites behind us, nobody can beat us."
But Mr Trump evaded questions about how he would carry out his campaign promises, especially his boast that he would build a wall along the Mexican border to stop illegal immigration and make Mexico pay for it.
He did concede that he is not doing as well with women voters because of the stinging language of his campaign.
"I can see women not necessarily liking the tone (of his campaign), but I had to be very harsh to win," he said of his brutal counter-attacks on party elites and fellow candidates, particularly Florida senator Marco Rubio and Texas senator Ted Cruz.
Mr Trump's lead over Mr Cruz in the race for delegates grew by only 15 delegates. That is because all four states awarded delegates proportionally, so even the second-place finisher got some.
Mr Cruz captured the Idaho primary in the west of the country. Mr Rubio, the favourite of the party establishment, failed to pick up any delegates on Tuesday. He needs to win home state Florida next week, while Ohio governor John Kasich needs to win his home state Tuesday to stay in the race.
Among Republicans, Trump has at least 446 delegates and Cruz has at least 347. Rubio has at least 151 delegates and Kasich has at least 54. It takes 1,237 delegates to win the party nomination.
While Mr Sanders upset Mrs Clinton in Michigan, she increased her delegate lead by sweeping Mississippi and is now halfway to the number needed to clinch the nomination.
After Tuesday's results, Mrs Clinton has accumulated 1,221 delegates and Mr Sanders 571, including superdelegates, the party insiders who can support whoever they like. Democrats need 2,383 delegates to win the nomination.
Mr Sanders said that "in almost all national polls" he is the bigger winner against Mr Trump in the election.
Mrs Clinton chose to focus her attention on Republicans and the election. "We are better than what we are being offered by the Republicans," she declared.
The economy ranked high on the list of concerns for voters in Michigan and Mississippi. At least eight in 10 in each party's primary said they were worried about where the American economy is heading, according to exit polls conducted by Edison Research for The Associated Press and television networks.
Among Democrats, eight in 10 voters in both states said the country's economic system benefits the wealthy, not all Americans.
Mr Sanders has sought to tap into that concern, energising young people and white, blue-collar voters with his calls for breaking up Wall Street banks and making tuition free at public colleges and universities.
But Mr Sanders has struggled to win the support of black voters who are crucial to Democrats in the general election.