The House panel investigating last year's attack on the Capitol on Tuesday made public a chain of texts showing Riley's attempts to set up a hand-off between the senator and vice president.
"Johnson needs to hand something to VPOTUS please advise," Johnson's aide texted Pence aide, Chris Hodgson, shortly before Congress convened in a joint session to certify the election results.
When Hodgson asked what it was, Riley texted: "Alternate slate of electors for MI and WI because archivist didn't receive them."
"Do not give that to him," Hodgson replied tersely.
The revelation outraged Johnson's Democratic colleagues, who say that his staff may have broken the law by attempting to submit a false set of electors to Pence.
"Absolutely appalling if that took place," said Sen. Jeff Merkley (D-Ore.). "I hope the details are fully explored by the press in a way that people get their hands around and understand how significant that is when you start trying to substitute electors from what a state has certified and submitted."
Sen. Debbie Stabenow (D-Mich.), whose state's duly elected electors might have been thrown out if Pence accepted and deployed the contents of the envelope, said trying to overturn the results of a legitimate election would be a violation of law.
"First of all, it's horrifying that they actually had a separate group of electors and thought that was okay. I don't know what happened with Sen. Johnson but he certainly has promoted the ‘Big Lie,'" Stabenow said.
Asked whether an Ethics Committee investigation is warranted, Stabenow said, "It is very serious. We'll leave it to the Ethics Committee."
But she added "it's serious if he in anyway was acknowledging any of this as legitimate."
"Fake electors literally would be an effort to overturn the government," she added.
Johnson said Democrats are already attacking him over the attempted hand-off.
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Wisconsin Lt. Gov. Mandela Barnes, who is running in the Senate Democratic primary to challenge Johnson in November, accused the Republican senator of trying to undermine democracy.
"He literally tried to hand Mike Pence fake ballots. Once again, Ron Johnson has proven he's a danger to our country and our fundamental rights. I'm calling for him to resign immediately," Barnes said.
Philip Schulman, a spokesperson for the Democratic Party of Wisconsin, said instead of focusing on what's best for Wisconsinites, "Johnson's focus in Washington is working to undermine democracy."
A Marquette Law School Poll of 803 registered Wisconsin voters released on Wednesday showed Barnes leading Johnson 46 percent to 44 percent in a hypothetic matchup.
The same poll showed Johnson trailing Democratic candidate Sarah Godlewski by two points and narrowly beating Democratic candidate Alex Lasry by three points.
Brandon Scholz, a Wisconsin-based Republican strategist, however, predicted the impact of the Jan. 6 committee's revelation will have little impact on the outcome of the Senate race.
"Not many people care," he said of the news coming out of the Jan. 6 committee's hearings. "It really is kind of a D.C. show. Whether or not this has any traction amongst voters here, today I would tell you no."
Scholz said there's "so much content" in the Jan. 6 committee's investigation that "it bounces off people."
He acknowledged that Democratic voters are going to be fired up over the revelation "but they're not going to vote for Ron Johnson anyway."