Republican presidential nominee Donald Trump is doubling down once again, this time on his claims that U.S. voter fraud is widespread. “Of course there is large scale voter fraud happening on and before election day,” Trump tweeted Monday morning. “Why do Republican leaders deny what is going on? So naive!” Trump has recently amped up his rhetoric about the election being “rigged” and potentially stolen from him, without providing evidence as to how that would be the case. He added in a subsequent tweet: “We have all got to come together and win this election. We can’t have four more years of Obama (or worse!).” On Sunday and again on Monday, a spokeswoman for House Speaker Paul Ryan pushed back on Trump’s claims of widespread voter fraud, saying that Ryan believes the election will be carried out fairly.