WASHINGTON (REUTERS) - US Republican presidential candidate Donald Trump went on a charm offensive to try to win the party establishment's support for his insurgent candidacy, but top Republican Paul Ryan stopped short of endorsing him.

Mr Trump was on his best behaviour on a day of meetings with Republican lawmakers on Capitol Hill on Thursday (May 12). He listened patiently as they raised concerns about his tone and the need to try to appeal to Hispanic voters.

He avoided strident language like the frequent criticism he has lobbed from the campaign trail, that many lawmakers are awestruck by the corridors of power and forget why they were sent to Washington.

"The whole discussion was very solid, reasonable and a warm and winning discussion," said US Senator Orrin Hatch of Utah."I think you're going to find he's going to be better and better all the time."

Mr Trump's day in Washington was aimed at laying to rest some of the concerns that persist among Republicans about his incendiary tone and some policy proposals that violate party doctrine.

The New York billionaire, who needs the party behind him in order to have a chance at winning the Nov 8 election, has vowed to build a wall along the US border with Mexico, deport 11 million illegal immigrants, temporarily ban Muslims from entering the country and impose trade protectionist policies.

Mr Trump held an hour-long session with Mr Ryan, who as speaker of the House of Representatives is the top US elected Republican and can hold sway with many establishment Republicans leery of Mr Trump.

"This was our first meeting, but it was a very positive step toward unification," Mr Ryan and Mr Trump said in a joint statement.

The usually loquacious Mr Trump was restrained, issuing a tweet in which he said: "Things working out really well!" before flying home to New York.

Party leaders are normally eager to rally around a presidential nominee to combine forces for the battle leading up to the general election. But Mr Ryan has withheld his endorsement of Mr Trump out of concern over the businessman's conservative credentials.

In remarks to reporters after the meeting, the congressman said he was encouraged by the session but that more work will be needed.

"There's no secret that Donald Trump and I have had our differences. We talked about those differences today," Mr Ryan said at his weekly news conference. "I do believe we are planting the seeds in getting ourselves unified."

Mr Ryan, who may harbour aspirations of running for president in 2020 or later, noted that he represents a wing of the conservatives and that it is positive that Mr Trump is bringing new voters into the party.

Despite his problems in winning over senior Republicans, Mr Trump received a boost on Wednesday when a Reuters/Ipsos national poll showed him pulling even with likely Democratic nominee Hillary Clinton. The online survey found 41 per cent of likely voters supporting Mr Clinton and 40 per cent backing Mr Trump.

Later on Thursday, Mr Trump went into a meeting with Senate Republican leaders, where he posed for photos with them and heard concerns about his campaign rhetoric but appeared to make some progress in tempering concerns about him.

"Everyone here wants you to win," Senate Majority Leader Mitch McConnell told him at the Senate session, a source said.

US Senator Shelly Moore Capito of West Virginia urged Mr Trump to be careful in his tone. US Senator Rob Portman of Ohio, a former US Trade Representative, urged caution on his rhetoric against trade deals.

"The issue of tone did come up," said US Senator John Cornyn of Texas, who said he gave some advice to Mr Trump on "the importance of the Hispanic vote and the whole idea of distinguishing between illegal immigration and legal immigration".

In a meeting at a Washington law firm, Mr Trump sat down with Mr James Baker, who served as secretary of state for Republican President George H.W. Bush.

Earlier in the day, Mr Baker had testified to the Senate Foreign Relations Committee. Under questioning from US Senator Marco Rubio, a former Republican presidential candidate, Mr Baker said the world "would be far less stable" without a strong Nato, a slap at Mr Trump's idea of reconfiguring the Western alliance and getting European nations to foot more of the bill.

"Secretary Baker had a meeting with Donald Trump that was requested by his campaign," a Baker spokesman said.

Even Senator Lindsey Graham of South Carolina softened a bit. He dropped out of the presidential race earlier this year and had said the choice between Mr Trump and rival Ted Cruz was like trying to decide between being "shot or poisoned".

He said he had a "cordial, pleasant conversation" on the phone with Mr Trump on Wednesday. "I know Mr. Trump is reaching out to many people, throughout the party and the country, to solicit their advice and opinions.

"I believe this is a wise move on his part," said Mr Graham.