(CNN) Republicans are trying to embrace Donald Trump -- but he isn't making it easy.

Just three weeks before the party plans to coronate Trump as its 2016 standard-bearer at the Republican National Convention in Cleveland, the presumptive nominee is keeping up the infighting that has troubled the GOP's establishment for months.

"It's almost -- in some ways, like, I'm running against two parties," Trump told conservative talk radio host Mike Gallagher on Thursday.

In the past two days, Trump has abandoned decades of conservative orthodoxy on trade, launched into a battle with the GOP's traditional business-lobby allies and campaign financiers -- like the Chamber of Commerce -- and slammed his former Republican presidential rivals who haven't endorsed him, saying their political careers should be over.

"They broke their word and in my opinion, they should never be allowed to run for public office again because what they did was disgraceful," Trump said in Bangor, Maine, Wednesday, alluding to figures like Florida Gov. Jeb Bush, Texas Sen. Ted Cruz and Ohio Gov. John Kasich, who signed a pledge to support the GOP nominee but have yet to endorse him. Kasich, for his part, on Wednesday released a letter on his campaign fundraising list highlighting a poll showing him faring better in the general election than Trump.

Trump will continue to make his case against free trade deals like the Trans-Pacific Partnership, negotiated under President Barack Obama, and the North American Free Trade Agreement, signed into law by Bill Clinton, in a speech Thursday afternoon in Manchester, New Hampshire.

On style and substance, Trump is again showing many of the qualities that have left Republicans on Capitol Hill and in the party's traditional donor community hesitant to embrace him. Just 74% of Republicans back him over Hillary Clinton, a Fox News poll released Wednesday showed , compared to 82% in May. In 2012, GOP nominee Mitt Romney earned 93% support among Republicans.

The latest infighting comes as Republicans attempt to come to grips with Trump's looming nomination.

Senate Majority Leader Mitch McConnell, R-Kentucky, said Trump is "getting closer" to becoming a credible candidate in an interview with NY1/Time Warner Cable News.

"My hope is that he is beginning to pivot and become what I would call a more serious and credible candidate for the highest office in the land," McConnell said.

But the GOP establishment's embrace is still only tepid.

Utah Sen. Mike Lee on Wednesday cited Trump's use of a baseless tabloid report that Cruz's father helped conspire to kill President John F. Kennedy -- among other things -- as a reason he has yet to endorse the presumptive Republican nominee in a heated exchange with a reporter.

"What I am saying is Donald Trump can still get a vote from a lot of conservatives like me, but I would like some assurances on where he stands," Lee said. "I would like some assurances that he is going to be a vigorous defender of the U.S. Constitution. That he is not going to be an autocrat. That he is not going to be an authoritarian."

Arizona Sen. Jeff Flake, who also has not endorsed Trump, said Trump has "shown these signs before and snapped right back to the old Donald. So it's too early to tell."

In particular, Flake said, Trump's protectionist opposition to trade deals is "troubling."

"Very bothersome and very damaging, frankly, for the long term," Flake said.

On trade, Trump's stance could be closer to where the Republican Party's electorate is in 2016 than the GOP's pro-trade nominees of years' past.

His fierce opposition to free trade deals has the effect of making the Republican agenda -- at least while he's setting it -- more accurately reflect his coalition. The people who support Trump are older, whiter and less educated, which means they've suffered the most from these trade deals -- and it's not as easy for them to go back to school or change their retirement plans.

However, the American electorate is becoming younger and less white. That's why down-ballot Republicans are concerned that Trump's coalition isn't a winning one for the party.

If Trump has his way, the fights with the Chamber of Commerce, the White House and fellow Republicans will set 2016's stakes as a referendum on the political status quo -- no matter the party in power.

"If we're going to deliver real change, we're going to have to reject the campaign of fear and intimidation being pushed by powerful corporations, media elites, and powerful dynasties," Trump said Tuesday in his economic speech in Rust Belt Pennsylvania. "The people who rigged the system for their benefit will do anything and say anything to keep things exactly as they are."

Trump's position on trade isn't just a talking point -- it's central to even his warm-up acts at campaign rallies, which veer harder than even Trump himself toward nationalism.

"Folks, we've been following that economic advice year after year after year, and what do we have to show for it?" senior Trump policy adviser Stephen Miller told the crowd in Maine. "Maybe it's time we stop taking lectures from the people who've impoverished the middle class in this country."

Miller tagged Clinton as "the embodiment of globalism" and "financial elitism."

"In January, we're going to declare our independence from globalism and we're going to declare our independence from the politicians who would oppress us all under its mantle," he said.

His campaign has identified the issue as a winner, hoping Trump can perform strongly enough with blue-collar workers in manufacturing-heavy states like Ohio, Pennsylvania and Michigan to offset potential losses in states with larger Latino populations like Florida, Colorado, Nevada and Arizona.

It's also an issue that puts Clinton in an awkward position. On Tuesday, Clinton's campaign responded to Trump's attack on trade deals by casting his positions as "surprisingly familiar."

As in, similar to Clinton's positions.

Instead of criticizing Trump's position, Clinton's campaign noted that the former secretary of state has a longer history fighting Chinese currency manipulation and pushing for strict enforcement of trade rules already on the books.

"While Hillary Clinton was fighting for stronger trade enforcement and American manufacturing, and going toe-to-toe with China, Donald Trump was outsourcing many of the goods that he sold, instead of making it in America," Clinton's campaign said in the email.

Trump, meanwhile, spent part of Wednesday fighting against the U.S. Chamber of Commerce and the National Association of Manufacturers, two key business groups that have funneled millions into Republican campaigns in recent years.

He told the Mainer crowd the Chamber should embrace him because he's willing to support free trade deals -- as long as they're better-negotiated than any currently on the books.

"I'm all for free trade. The problem with free trade is you need smart people making deals. We don't have good deals. And free trade is killing us," he said.

Trump compared his position on trade not to other Republicans, but to Clinton's primary rival, Vermont Sen. Bernie Sanders.

"There is one thing that Bernie Sanders and I are in complete accord with and that's trade," Trump said. "He said we're being ripped off and I say with being ripped off I've been saying it for years he's been saying it for years. I think I am saying it even louder."