President Trump continued on Sunday his attack on mail-in balloting and other alternatives to in-person voting as the final month before the presidential election nears.

Speaking during a press briefing at the White House, Trump tore into what he called,“Democrats' attacks on election integrity,” specifically calling out instances of missing or dumped ballots in places like Pennsylvania and New Jersey.

"Joe Biden and the Democrat extremists oppose all of these efforts to ensure election integrity,” Trump said. “They want none of the safety valves …They want all states to do mail-in balloting…They want unsolicited mail-in balloting.”

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Trump specified a case in Pennsylvania’s Luzerne County where nine mailed-in military ballots were “discarded” by the local election office. Officials so far have released little information in that case, including whether the ballots were blank and on their way to voters, or if they were completed and being returned to the local election office.

Trump, after he was briefed on the case by Attorney General William Barr, said in an interview last Thursday with Fox News Radio that the discarded ballots were cast for him.

A subsequent investigation blamed an independent contractor hired to assist with the increased demand for mail-in ballots. The nine ballots, from members of the military, were initially reported by the U.S. Attorney's Office to all have been cast for Trump.

But a later release said seven were for Trump, and two were resealed in envelopes for submission and the preference was not known.

“The ballot and the system is rigged,” Trump said Sunday.

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Mail-in balloting has been a particular point of ire for the president in the run-up to November’s election, with Trump making claiming numerous times that the practice is rife with fraud.

Election experts say voting by mail is more susceptible to fraud than casting a ballot in person, but they’ve seen no evidence of widespread fraud or that absentee balloting favors Democrats. However, the massive increase in absentee balloting places an extra burden on already stressed state and county election officials and on a U.S. Postal Service facing financial and manpower deficits.

Due to concerns over the coronavirus pandemic, more than half of Americans are expected to vote through the mail this presidential election. More than 40 percent of Americans voted by mail in the 2016 and 2012 general elections, according to the Election Assistance Commission.

Fox News’ Paul Steinhauser and the Associated Press contributed to this report.