Conspiracy theories aside, I recall one recent proven case of fraud where the result might have been affected by it, and it was committed by a Republican candidate:

"In 59 Philadelphia voting divisions, Mitt Romney got zero votes"

https://www.inquirer.com/philly/news/politics/20121112_In_59_Philadelphia_voting_wards__Mitt_Romney_got_zero_votes.html



"Statement by the Press Secretary on the Presidential Advisory Commission on Election Integrity"

https://www.whitehouse.gov/briefings-statements/statement-press-secretary-presidential-advisory-commission-election-integrity/



"A Sampling of Recent Election Fraud Cases from Across the United States"

https://www.heritage.org/voterfraud



"| The Problem of Duplicate Voting 20171America The Vulnerable:  The Problem of Duplicate Voting"

http://g-a-i.org/wp-content/uploads/2017/07/Voter-Fraud-Final-with-Appendix-1.pdf



"Voter Fraud: We’ve Got Proof It’s Easy"

https://www.nationalreview.com/2014/01/voter-fraud-weve-got-proof-its-easy-john-fund/



"Democrats Claim Voter Fraud is a Myth; Here's Proof It's Not"

https://thefederalistpapers.org/us/democrats-claim-voter-fraud-is-a-myth-heres-proof-its-not



"Proof Democrats are in Conspiracy to Commit Voter Fraud"

https://www.investmentwatchblog.com/proof-democrats-are-in-conspiracy-to-commit-voter-fraud/



"Colorado Counties Have More Voters Than People"

https://www.redstate.com/aarongardner/2012/09/04/colorado-counties-have-more-voters-than-people/



"In Florida: Obama Got Over 99% in Broward County Precincts "

www.punditpress.com/2012/11/in-florida-obama-got-over-99-in-broward.html



"St. Lucie County, Florida Had 141.1% Turnout; Obama Won County "

www.punditpress.com/2012/11/breaking-st-lucie-county-florida-had.html



"Obama Won Dozens of Cleveland Districts with 100% of the Vote "

www.punditpress.com/2012/11/what-luck-obama-won-dozens-of-cleveland.html



"Is voter fraud being committed in Ohio?"

https://humanevents.com/2012/10/26/is-voter-fraud-being-committed-in-ohio/



"Florida Vote Twist: More Ballots Than Voters"

https://www.washingtontimes.com/blog/robbins-report/2012/nov/10/florida-vote-twist-more-ballots-voters/



"Voter rolls in Ohio are bloated, experts say"

https://www.dispatch.com/content/stories/local/2012/09/16/voter-rolls-in-ohio-are-bloated-experts-say.html



"'You have to vote for Dems,' NYC poll worker tells ... GOP Senate candidate"

https://www.foxnews.com/politics/you-have-to-vote-for-dems-nyc-poll-worker-tells-gop-senate-candidate



"Claims increase of machines switching votes in Ohio, other battlegrounds"

https://www.foxnews.com/politics/claims-increase-of-machines-switching-votes-in-ohio-other-battlegrounds



"Poll watcher sees Romney ballots changed"

https://www.wnd.com/2012/11/poll-watcher-sees-romney-ballots-changed/



"Vote Harvesting a Recipe for Coercion and Election Fraud"

https://www.heritage.org/election-integrity/commentary/vote-harvesting-recipe-coercion-and-election-fraud



"Database Swells to 1,285 Proven Cases of Voter Fraud in America"

https://www.heritage.org/election-integrity/commentary/database-swells-1285-proven-cases-voter-fraud-america



"Illegal Ballot Harvesting a Looming Issue in Arizona"

https://www.realclearpolitics.com/articles/2019/02/22/illegal_ballot_harvesting_a_looming_issue_in_arizona_.html



"HIDDEN CAM: NYC Democratic Election Commissioner, "They Bus People Around to Vote""

https://www.youtube.com/watch?v=jUDTcxIqqM0



"North Carolina vote fraud case shows just why Congress should ban ballot-harvesting nationwide"

https://www.americanthinker.com/blog/2019/02/north_carolina_vote_fraud_case_shows_just_why_congress_should_ban_ballotharvesting_nationwide.html



"Michigan Recount Uncovers Serious Voter Fraud in Detroit- VOTES COUNTED UP TO 6 TIMES"

https://www.thegatewaypundit.com/2016/12/revealed-michigan-recount-uncovers-massive-voter-fraud-detroit-precincts/



"Mass Detroit Voter Fraud? - 37% Of Precincts Counted More Ballots Than Voters"

https://www.zerohedge.com/news/2016-12-14/mass-detroit-voter-fraud-37-detroit-precincts-tallied-more-ballots-voters



"Hacking Democracy" (HBO)

https://www.youtube.com/watch?v=6YldIdkjrqM