What political group gained power after the 1876 presidential election?

The election of 1876 was a landmark election, which transferred power to the Democrats. After the election, no less than 20 electoral votes were disputed, making it unclear whether Rutherford B. Hayes or Samuel J. Tilden won the election. Florida, Louisiana, and South Carolina reported different winners than the ballot count, prolonging the election results. In the end, Hayes was declared the winner.

So, in short, your answer is Democrats