Georgia Republican Johnny Isakson’s resignation adds another seat to the 2020 Senate battleground and gives Democrats another takeover opportunity in their road to the majority.

According to state law, GOP Gov. Brian Kemp will appoint a senator, who will then stand for election in November 2020 to fill the remainder of Isakson’s term. Isakson was most recently reelected in 2016, 55 percent to 41 percent, and would have been up again in 2022.

Georgia’s other senator, Republican David Perdue, was already scheduled to be on the ballot in 2020. His race is currently rated Leans Republican.

It’s usually a good idea to know who is running before rating a race. But because of the high correlation of results when a state hosts two Senate elections at the same time, replicating the rating for Perdue’s seat for the new seat, at least initially, is the most logical thing to do.

Over the last 70 years, there have been 29 times when both of a state’s Senate seats were up for election. In 26 of those instances (90 percent of the time), the same party won both seats.

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In the most recent example, during the 2018 midterms, Democrats held both of Minnesota’s Senate seats, while Republicans held both seats in Mississippi.

The parties splitting the results is more rare.

In two of the three instances when candidates from different parties won concurrent Senate races in the same state, the split results maintained the partisan status quo before the election.

In Idaho in 1962, Democratic Sen. Frank Church won reelection, while appointed Sen. Len Jordan’s victory retained the Republican seat. And in South Carolina in 1966, Republican Sen. Strom Thurmond won reelection, just as Ernest F. Hollings held the Democratic seat after defeating the incumbent in the primary.

The third case of a split result was in New Hampshire in 1962, when Republican Sen. Norris Cotton won reelection, while his party lost the state’s other seat. The senator who had been appointed to fill that vacancy and who ran to fill the rest of the unexpired term, Maurice J. Murphy Jr., lost in the Republican primary, and Democrat Thomas J. McIntyre defeated GOP Rep. Perkins Bass in the general election.

In Georgia this cycle, Democrats had already struggled to find a top-tier challenger to Perdue. Now they’ll need to find two credible candidates in order to take advantage of this new opportunity. For now, both races are rated Leans Republican.