Proof is for the courts.

It's all about perception. If many think elections are rigged then this is very effective voter suppression. "Why bother voting if it's rigged."

Excerpt:
Much of the impeachment case against former President Trump in this week’s Senate trial has focused on his claims about election fraud, but a majority of Republican voters agree with his claim that Joe Biden was not elected fairly.

A new Rasmussen Reports national telephone and online survey finds that 57% of Likely U.S. Voters say Biden won the 2020 presidential election fairly, but 61% of Republicans say Biden did not win the election fairly. (To see survey question wording, click here.)

The impeachment case presented by House Democrats in the Senate trial accuses Trump of making false claims about election fraud that incited his supporters to riot at the Capitol on January 6. But 34% of voters say Biden did not win the election fairly, including 36% of voters not affiliated with either major political party.

(Emphasis mine)

Most GOP Voters Still Don’t Think Biden Was Elected Fairly
https://www.rasmussenreports.com/public_content/politics/elections/election_2020/most_gop_voters_still_don_t_think_biden_was_elected_fairly