Trump made the remark at the end of a speech to supporters in Oshkosh, Wis., on Monday afternoon as he implored them to vote. It follows months of Trump claiming without evidence that widespread mail-in voting during the coronavirus pandemic would invite fraud into the election.

“We have to win the election. We can’t play games. Go out and vote. Do those beautiful absentee ballots, or just make sure your vote gets counted. Make sure because the only way we’re going to lose this election is if the election is rigged,” Trump told the group of supporters at the outdoor campaign event.

“Remember that. It’s the only way we’re going to lose this election, so we have to be very careful.”

“The only way they’re going to win is that way. And we can’t let that happen,” Trump said just hours before Democrats are set to begin their 2020 convention to formally nominate Joe Biden as the party’s presidential candidate, The Hill reported.

Biden leads Trump nationally and in a number of key swing states, according to recent polling.

Democrats are increasing their scrutiny of the US Postal Service after it warned states last week that it could not guarantee all ballots cast by mail for the November election would arrive in time to be counted even if voters follow state guidelines.

The disclosure came after Postmaster General Louis DeJoy adopted several changes to the agency, such as eliminating some mail-sorting machines that have attracted criticism. DeJoy has cast the moves as cost-cutting measures meant to help the agency, which has long faced financial problems.

Trump has backed DeJoy, a Republican megadonor, while denying that his administration is undermining the agency.

“It’s been run horribly. And we’re going to make it good,” Trump said in an interview on “Fox & Friends” on Monday morning. “Now what am I supposed to do? Let it continue to run badly? So if you fix it, they say, ‘Oh, he’s tampering with the election.’ No, we’re not tampering.”

House Democrats intend to come back to Washington on Saturday to vote on legislation to block the measures implemented by DeJoy and ensure the agency can handle an expected flood of mail-in ballots ahead of the November election.

Democratic leaders have pressed for billions in additional funding for the Postal Service in the next coronavirus relief bill, but talks with the White House have broken down. DeJoy is also slated to testify before the House Oversight and Reform Committee next Monday.

Trump suggested last week that he was opposed to additional Postal Service funding because it would help implement universal mail-in voting this fall but hours later shifted his position by saying he would approve funding if Democrats made other concessions sought by the White House in coronavirus relief legislation. The president has tried to blame Democrats for a lack of funding for the Postal Service to assist the election.

Trump and his campaign have largely dismissed the surveys showing him trailing Biden, arguing they understate the president’s support as others did during the 2016 cycle.

Trump’s warning about the election being “rigged” punctuates a broader assault on mail-in ballots that the president has waged with fervency since states began efforts to expand mail-in voting so that individuals can cast ballots without having to physically go to the polls during the pandemic.

Experts say, however, that there is scant evidence of meaningful voter fraud associated with mail-in ballots, and critics have viewed the president’s attacks as an effort to sow distrust about the election results.

Late Monday, Trump retweeted the tweet from an account with the handle “@stella_immanuel” that said: “Covid has cure. America wake up.”

Twitter soon after removed the tweet and replaced it with a gray box that says, “This Tweet is no longer available.”

A cure for COVID-19, the disease caused by the coronavirus, doesn’t exist, and scientists have been working on developing both a range of treatments and vaccines. They and the Trump administration are racing to have a vaccine ready by the end of the year.

Twitter said early Tuesday, “Tweets with the video are in violation of our COVID-19 misinformation policy.”

Trump also retweeted tweets defending the use of the drug hydroxychloroquine, including one that accused Dr. Anthony Fauci, a member of the White House coronavirus task force, of misleading the public by dismissing the drug.

“I have not been misleading the public under any circumstances,” Fauci responded Tuesday morning on ABC’s “Good Morning America” when asked for his reaction to Trump’s retweets.

Fauci reiterated that the “overwhelming prevailing clinical trials” that have looked at the efficacy of hydroxychloroquine have indicated that it’s “not effective” in treating the coronavirus, NBC News reported.

Meanwhile, the US president’s eldest son, Donald Trump Jr., was temporarily locked out of his Twitter account on Tuesday after posting the same controversial video shared by Trump promoting hydroxychloroquine and saying people don’t need to wear masks, said Andy Surabian, an adviser to the president’s son. In a text message, Surabian said the president’s son should be able to resume tweeting later in the day.

“Twitter suspending Don Jr. For sharing a viral video of medical professionals discussing their views on Hydroxychloroquine is further proof that Big Tech is intent on killing free expression online and is another instance of them committing election interference to stifle Republican voices,” Surabian said in a statement. “It is beyond the pale for Twitter to silence someone for sharing the views of medical professionals who happen to dissent with their anti-Hydroxychloroquine narrative.”

The president had recently tried to take a more moderate approach to the COVID-19 pandemic.

He conceded, for example, last week at one of his restarted coronavirus White House briefings that it will likely “get worse before it gets better.” The president has also been touting the importance of wearing masks after dismissing them for months. Despite his change in tone, he pushed states to reopen on Monday as a number of experts are calling for states to shut down.

Twitter has been much more aggressive in recent months in either removing tweets posted by the president, fact-checking them in real time or even placing warning labels over them.

The social media giant first fact-checked a pair of tweets posted by Trump in May when he claimed there was “NO WAY” an election with boosted mail-in voting would be legitimate. Twitter added a fact-check label to both tweets.

A few days later, Trump signed an executive order that asked federal regulators to revisit a 1996 law that protects websites from liability for what their users post.

And then a day after he signed the executive order, Twitter put a tweet from Trump behind a warning label, which said that the president had violated its rules against glorifying violence when he tweeted about protests over the death of George Floyd.

In June, Twitter and Facebook removed a video posted to Trump’s social media accounts of a “racist baby” because of a copyright claim from one of the children’s parents. Twitter labeled the video “manipulated media.”

Trump stood by his comments on hydroxychloroquine at a press briefing later Tuesday, telling reporters “I happen to believe in it.”

“I would take it. I, as you know, took it for a 14-day period, and I’m here,” Trump said. “It didn’t get me, and it’s not going to — hopefully — hurt anybody.”

The Food and Drug Administration has recommended against taking hydroxychloroquine or chloroquine to treat or prevent COVID-19. Trump said in May that he was taking hydroxychloroquine to prevent contracting the virus.

Trump abruptly ended the briefing Tuesday after a reporter asked him why he would share content to his Twitter account from a doctor who has pushed wildly false medical information including that doctors make medicine using DNA from aliens and that doctors are trying to create a vaccine to make people immune from becoming religious.

“She was on air along with many other doctors. They were being fans of hydroxychloroquine, and I thought she was very impressive in the sense that — from where she came, I don’t know which county she comes from — but she said that she’s had tremendous success with hundreds of different patients, and I thought her voice was an important voice. But I know nothing about her,” Trump said before leaving the briefing room.