I haven't seen evidence of millions but it did happen in Nevada in 2016.
It seems that a legal challenge mooted then might not have happened?"These Republican plans to interfere with the integrity of Nevada’s Democratic caucuses are shameful and immoral," Reid stated, according Politico. "Rather than letting voters decide and allowing our democratic system to work, Republicans are resorting to trickery and gimmicks in an attempt to subvert the will of the people."
"The Republican Party has long decried voter fraud, but with this latest scheme they are now encouraging it," the statement continued. "The American people deserve a fair voting process, and I will do everything in my power to ensure that these disgraceful Republican tactics do not interfere with the voice of Nevada voters."
Roberta Lange, the head of Nevada's Democratic party threatened legal action in her statement:“After reviewing Nevada law, we believe that registering under false pretenses in order to participate in the Democratic caucuses for purposes of manipulating the presidential nominating process is a felony. The Nevada State Democratic Party will work with law enforcement to prosecute anyone who falsely registers as a Democrat to caucus tomorrow and subsequently participates in the Republican caucuses on Tuesday.”
“After reviewing Nevada law, we believe that registering under false pretenses in order to participate in the Democratic caucuses for purposes of manipulating the presidential nominating process is a felony. The Nevada State Democratic Party will work with law enforcement to prosecute anyone who falsely registers as a Democrat to caucus tomorrow and subsequently participates in the Republican caucuses on Tuesday.”"
https://archive.attn.com/stories/6073/republicans-for-bernie-dem-caucus
