That's a very questionable conclusion. While we know they used horrific means to reclaim control (reference the situation in Wilmington, NC where Democrats employed a literal coup d'etat to overthrow the biracial GOP city government), I see no definitive proof they murdered hundreds of Black elected officials. I have no doubt White supremacists who supported Democrats murdered or drove out Blacks who voted Republican, but had there been a situation where elected officials were being murdered en masse, there would've been a stepping-up of federal occupation by the military and an extension of Reconstruction. That would've been intolerable.
"All of the elected officials up to 1935 were Republicans."
This is also not entirely correct. Now while all Black federal Republicans were until 1935, you had many areas that had Black Democrats as well serving at the legislative and city levels. I was astonished to find in TN at least one Black Democrat in the legislature in Reconstruction (I'd surely like to know more about him and how he was able to pull it off !). You also had a lot of patronage positions as well. Even into the 1930s, Blacks were not voting GOP at the percentage they do now for Democrats. Some areas managed to get 1/4th to vote Democrat. If we got that high a percent today, we'd have a lot more GOP officials.
"As of 2004, the Democrat Party (the oldest political party in America) has never elected a black man to the United States Senate, the Republicans have elected three."
It was Zero who became the first Black rodent Senator. One minor correction here, we elected 4 GOP Senators (Hiram Revels (MS) in 1870, Blanche Bruce (MS) in 1875, PBS Pinchback in LA, and Ed Brooke in 1966). Pinchback, who was the first Black Governor of any state, was elected to both the House and Senate after he left the Governorship, but was denied his seat in both.