(CNN) On Monday, the Supreme Court rejected a Republican effort to halt a required redrawing of the state's 18 congressional districts before the year's midterm elections.

That is a very big deal. Here's why.

Republicans have controlled the line-drawing process in Pennsylvania for the last two decades. That control has created a congressional map that heavily favors Republicans -- they control 13 seats to the Democrats' five -- despite the fact that the state is a Democratic leaning one. (Before Trump carried Pennsylvania in 2016, George H.W. Bush in 1988 was the last GOP presidential candidate to win the state.)

Earlier this year, the Pennsylvania Supreme Court said the map was drawn primarily for political purposes -- and, therefore, violated the state's constitution. The state Supreme Court ordered the state legislature, which is still controlled by Republicans, to make a new map for the 2018 election -- and have it approved by Democratic Gov. Tom Wolf.

Republicans had hoped to avoid that fate with their appeal. That hope ended at the Supreme Court today.