Donald Trump is testing out themes to use against Democrat Hillary Clinton as the presumptive US Republican presidential nominee tries to persuade disgruntled party loyalists to get behind his campaign.

Some top leaders within his party - US House Speaker Paul Ryan among them - are still expressing reluctance to rally behind him but Trump is hoping to entice more support from Republicans by highlighting their common opponent.

On Friday, Trump took aim at Clinton for her use of a private email server while US secretary of state. Clinton has said she did not send or receive information marked as classified. The FBI is investigating whether laws were broken.

‘The email scandal should take her down but I don't think it's going to because I think she's being protected by the Democrats,’ Trump said on ‘Fox & Friends,’ a television news program that attracts a large conservative viewership.

Trump tried to cast Clinton and Democrats as facing more internal strife.

‘If you look at what she's going to do, she's going to be so bad on jobs that wages are going to go down for workers,’ he said.

Republican Chairman Reince Priebus has already begun his efforts to rally behind Trump, saying at an event Friday hosted by Politico that behind the scenes the candidate is ‘more gracious and personable than I think you see at rallies.’

‘I think there's work on tone to do,’ Priebus said, adding that he has made that point to Trump recently. ‘I think he gets it. I think you're going to see it. I think you're going to see the change in tone.’

In addition to changing tone, Trump also faces an uphill climb to bring the party together. Ryan, the top elected Republican in the United States, said on Thursday he was not ready to support Trump, a sign of lingering establishment concern about the candidate's position on immigration and trade.

Trump took to social media on Friday morning to continue to respond to Ryan.

‘Paul Ryan said that I inherited something very special, the Republican Party,’ Trump wrote on Twitter. ‘Wrong, I didn't inherit it, I won it with millions of voters.’

TRUMP SURPRISED BY RYAN

In the Fox interview, Trump said he was ‘very, very surprised’ by Ryan's reluctance.

‘I mean he talks about unity but what is this about unity?’ he said. ‘And with millions of people coming into the party, obviously I'm saying the right thing.’

Trump said he expected to meet with Ryan on Wednesday.

At a rally in West Virginia on Thursday night, the  billionaire businessman criticized Clinton for the vast sums of money that she and her husband, former President Bill Clinton, accepted for the Clinton Foundation, which he called a ‘scam.’ The Clintons have dismissed criticisms of the charitable organization as politically motivated.

Trump also linked her to some of her husband's decisions when he was president in the 1990s, such as the NAFTA agreement that opened up trade among the United States, Mexico and Canada. Trump has vowed to rewrite trade deals if elected on Nov. 8.

‘We cannot take Hillary Clinton anymore,’ he said. ‘NAFTA was given to us by Clinton. We can't take any more of the Clintons.’

He put on a hard hat presented to him by the state's coal miners' association and blasted Clinton for saying recently she would impose clean-energy policies that would put coal miners out of business.

In a sign some Republicans are rallying around Trump, Nebraska Governor Pete Ricketts, whose family has helped bankroll an anti-Trump group, is set to endorse him on Friday.

Former Texas Governor Rick Perry, who last year described Trump as a ‘cancer on conservatism’ while running against him for the Republican nomination, supported him as well.

Trump is looking at potential running mates and on Friday  ruled out picking a Democrat. He scoffed at the Republicans who have said they would not be interested in the job.

‘So many people are saying, 'I've decided to turn it down.' They were never on the list,’ he told Fox.

Trump's effort to raise as much as $1 billion for his campaign and the Republican Party for the general election is just getting started and got a boost on Thursday when billionaire casino owner Sheldon Adelson announced his support for Trump. Adelson has generously bankrolled candidates before, including donating $20 million to Newt Gingrich's failed 2012 campaign.

But Trump may not be able to rely on some major past sources of financial support.

One example of that is Theresa Kostrzewa, who raised money for 2012 nominee Mitt Romney and 2016 candidate Jeb Bush. She gave no indication she would raise money for Trump.

‘I'll be voting in November,’ she said. ‘Those are my plans.’