Republican U.S. presidential nominee Donald Trump attends a campaign rally at Blair County Convention Center in Altoona, Pennsylvania August 12, 2016. REUTERS/Eric Thayer

Republican Donald Trump should fix his stumbling White House campaign by Labor Day or step down, The Wall Street Journal said on Monday in a sharply worded warning from a leading conservative voice.

“Mr. Trump has alienated his party and he isn’t running a competent campaign,” the newspaper said in an editorial.

The paper’s editorial board, which generally favors Republicans, has been critical of Trump but its warning on Monday was its strongest attack yet and echoed growing anxiety among many Republicans over the state of Trump’s campaign.

The New York real estate developer, who has never held elected office, has been mired in weeks of controversy and opinion polls show him falling behind Democratic candidate Hillary Clinton in the race for the Nov. 8 election.

Trump’s “window for a turnaround is closing,” the Journal said, urging his backers to push the candidate toward a promised “pivot” to a presidential posture and a more disciplined campaign.

“If they can’t get Mr. Trump to change his act by Labor Day, the GOP will have no choice but to write off the nominee as hopeless and focus on salvaging the Senate and House and other down-ballot races,” it said.

Labor Day, which falls on Sept. 5 this year, marks the end of U.S. summer vacations and traditionally launches the final phase of the long U.S. election season.

“As for Mr. Trump, he needs to stop blaming everyone else and decide if he wants to behave like someone who wants to be president – or turn the nomination over to Mike Pence,” it said, referring to the Indiana governor, who is Trump’s vice presidential running mate

Trump has repeatedly provoked controversy in the weeks since his formal nomination as the Republican presidential candidate in July, despite appeals from party leaders for him to focus on issues that could win him the election.

He picked a fight with the parents of a Muslim U.S. Army captain who was killed in Iraq and falsely accused President Barack Obama and Clinton of being “co-founders” of Islamic State. He later said he was being sarcastic but has continued to repeat the remark.

Trailing in opinion polls in so-called battleground states, Trump has increasingly begun to portray himself as a victim of the media.

The Journal said Trump mistakenly believes his rowdy rallies will morph into votes and he can get away with relying on social media instead of spending money to compete in battleground states.

Manafort denied any impropriety in a statement on Monday. “I have never received a single ‘off-the-books cash payment’ as falsely ‘reported’ by The New York Times, nor have I ever done work for the governments of Ukraine or Russia,” he said.

The report came as Trump prepared to give a foreign policy speech on Monday. The Clinton campaign said it was evidence of “more troubling connections between Donald Trump’s team and pro-Kremlin elements in Ukraine.”

Trump has spoken favorably in the past of Russian President Vladimir Putin. Last month he invited Russian hackers to find “missing” emails from Clinton’s time as secretary of state, when she used a private computer server to conduct government business, although he later described that comment as sarcasm.

(Reporting by Doina Chiacu; Editing by Frances Kerry)