The legislature on Sunday pushed through the bill despite objections from Secretary of State Barbara Cegavske, the only statewide Republican elected official, on a party-line vote. It would give Democratic Gov. Steve Sisolak the power to command the secretary of state to adjust election procedures during a state of emergency.
The bill also expands who can turn in ballots, a provision many Republicans in the state said could open the door for ballot harvesting.
"In an illegal late night coup, Nevada’s clubhouse Governor made it impossible for Republicans to win the state," Trump said in a tweet Monday. "Post Office could never handle the Traffic of Mail-In Votes without preparation. Using Covid to steal the state. See you in Court!"
Trump, in a tweet Sunday, called the move by the Nevada legislature "outrageous" and said it should "be met with immediate litigation."
Democrats have dismissed concerns that universal mail-in voting could lead to fraud, saying there is little evidence that it has happened on a wide scale in the past. Meanwhile, Republicans have pointed out mail-in voting has seldom been conducted on the scale Democrats currently propose. They also distinguish between "absentee" voting, in which one has to request a ballot and usually provide a reason for being unable to vote on Election Day, and universal mail-in voting, in which the government sends ballots to every registered voter whether they have requested the ballot or not.
Attorney General Bill Barr, when pressed on the security of such voting during a House hearing last week, admitted that he had not seen evidence other countries could change the outcome of an American election using counterfeit ballots, but said it is "common sense" that it is a risk.
But more than other countries printing ballots, many on the right are concerned about the potential that ballots mailed to people who may have moved or are dead, yet still appear on voting rolls, could be fraudulently filled out.
Also, a CBS report last month showed that approximately 3 percent of mock ballots mailed in an experiment never arrived. In a statement on the report, the U.S. Postal Service said it is "committed to delivering election mail in a timely manner."
Nevada Senate Majority Leader Nicole Cannizzaro and Assembly Speaker Jason Frierson condemned Trump and Republican National Committee (RNC) Chairwoman Ronna McDaniel over statements each made on the mail-in measure.
"The comments made by President Trump and RNC Chairwoman Ronna McDaniel regarding Assembly Bill 4 are disgraceful and patently false," they said. "This is a clear fear mongering attempt by the GOP to suppress voters this November as part of an effort to shield themselves from the backlash of a failed administration."
They added that they are sending the bill to Sisolak's desk. He is expected to sign it, which would make Nevada the eighth state to opt for universal mail-in voting this November.
Mail-in voting is likely to be a contentious issue until the election, with Trump warning that it could cause the "greatest election disaster in history" and presumptive Democratic presidential nominee Joe Biden saying Trump is trying to "indirectly steal" the election by raising doubts about mail-in voting.
The escalating conflict over the Nevada bill comes on the heels of Trump last week suggesting that the Nov. 3 election could be delayed because of possible issues with mail-in voting -- comments the president and multiple senior members of his administration have since walked back.
The Associated Press contributed to this report.