WASHINGTON — Donald Trump’s decision to fire his embattled campaign manager less than a month before the Republican convention sent a powerful signal to weary GOP leaders that the billionaire businessman recognizes the increasingly dire state of his presidential campaign. Now, his party is looking for him to quickly implement other changes to mount a credible challenge to likely rival Hillary Clinton.

Trump’s decision to oust Corey Lewandowski aims to put an end to the months of infighting that had roiled his unconventional campaign. And it signals bigger changes are underway. Lewandowski, a constant presence at his boss’s side, had long resisted calls for Trump to adopt a more presidential tone and campaign operation, instead insisting the key to victory was to “Let Trump be Trump.”

That approach ultimately lost out to top GOP officials, donors and Trump’s adult children, who have long-harbored concerns about Lewandowski’s brusque manner and limited national campaign experience.

The shake-up comes as Trump is under growing pressure to build a credible general election campaign. Many in his party feel he has squandered the precious weeks since locking up the nomination. On the campaign trail, he appears stuck in a primary campaign he’s already won.

Indeed, fundraising reports released late Monday showed donors gave Trump’s campaign about $3 million in May, even though he enjoyed presumptive nominee status for almost the entire month. By contrast, Clinton, who spent all of last month fending off her primary rival, Bernie Sanders, raised $26 million. As a result, Trump’s  campaign started June with $1.3 million in the bank; Clinton has $42 million.