The GOP's best voter suppression tactic in states with large Black populations is not to run a candidate in Black districts against Black nominees. For example, in Houston, do not put up a GOP candidate against Sheila Jackson Lee.
The Dems try to motivate voters by hollering racism. Just take that card away. Sheila Jackson Lee will win anyway. But if the GOP runs a candidate against her the Dems will run through the Black neighborhoods asserting some mortal threat by White people against Blacks, and the need to ge defend Sheila and the threat to Blacks.
If a candidate has opposition they campaign more than if they are unopposed. All candidates do this. If a candidate like Sheila Jackson Lee has no personal reason to campaign, she'll campaign a lot less. It will affect voter turnout in her district. Voters who would come to vote for her, and then vote straight Dem while they're there, don't come. That affects candidates like O'Rourke.
One year I worked to support a candidate for a minority district in the state House. He had been a football star in the community (and played at Michigan) and was active in local politics. Promoting him drove up turnout in that district and probably cost us a key countywide race.
Another good voter suppression technique is to push an issue which splits the Dem electorate, forcing a candidate either to choose or wimp out. For years the gay marriage thing worked. It was popular with the true Left but not with Catholics, minorities or Billy Bob - all big Dem constituencies.