Groups calling for the Electoral College to reject President-elect Donald Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE are targeting electors with a $500,000 ad buy in the days leading up to the vote, according to USA Today.

The ad is an abbreviated version of a video released earlier this week that features celebrities like Martin Sheen and Debra Messing. It will air Saturday and Sunday on cable and satellite television in all 50 states, USA Today reported.

The ad, sponsored by Unite for America and Americans Take Action, is part of a last-ditch effort to persuade electors casting their votes Monday to reject Trump.

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The effort to rally so-called “Hamilton Electors” faces an uphill battle. So far just one Republican, Chris Suprun of Texas, has publicly said that he would back someone other than Trump.

Trump won 306 electoral votes last month, well beyond the 270 required to win the White House.

Despite media attention to scattered groups suggesting GOP opposition to Trump, there's little evidence suggesting many Republican voters will break from the popular vote in their home states to vote against Trump.