WASHINGTON — With just over a month to go before Americans head en masse to the polls in an extraordinarily contentious election, Facebook is expanding restrictions on political advertising, including new bans on messages claiming widespread voter fraud.

New prohibitions laid out in a blog post come days after President Donald Trump raised the prospect of mass fraud in the vote-by-mail process during a debate with Democratic rival Joe Biden.

Banned ads “would include calling a method of voting inherently fraudulent or corrupt, or using isolated incidents of voter fraud to delegitimize the result of an election,” Rob Leathern, Facebook’s director of product management, tweeted. The changes apply to Facebook and Instagram and are effective immediately, he said.

The ban includes ads that call an election into question because the result isn’t determined on the final day of voting. There is a good chance U.S. election results will require additional time this year because of expanded mail-in ballots due to the pandemic.

Also banned are advertisements portraying voting or census participation as meaningless and advising people not to take part.

The Trump administration has been trying to end the 2020 U.S. census early, as part of what opponents say is a strategy to enhance the political power of Republicans and non-Hispanic whites by undercounting minorities and others in hard-to-count communities. The census numbers are used to decide how many congressional seats each state gets for the next decade.

Facebook had unveiled a raft of measures this month to prevent its platform from being used to sow chaos and spread misinformation before, during and after the U.S. presidential election on Nov. 3. Last week it banned ads that prematurely declared an election victory. Before that, it said would restrict new political ads in the week before the election and remove posts that convey misinformation about COVID-19 and voting.