Sen. Johnny Isakson (R-Ga.) on Wednesday became the fourth GOP senator to announce a retirement ahead of the 2020 election, expanding the Senate map by an additional seat. And for Democrats, every potential pickup counts.

2020 Senate map Democratic-held Not up for election Republican-held NH ME MT OR MN ID SD MA WY MI RI IA NE NJ IL DE CO WV VA KS KY NC TN AZ OK AR NM SC GA AL MS LA TX AK Both of Georgia’s seats are up for election 50 22 12 35 Dems. not up 31 Reps. not up Democrats need to gain three seats to control the Senate, if they win the presidency 2020 Senate map Democratic-held Republican-held Not up for election NH ME MT OR MN MA ID SD RI WY MI IA NJ NE IL DE CO WV VA KS KY NC TN AZ OK AR NM SC GA AL MS LA TX Both of Georgia’s seats are up for election AK 50 35 Dems. not up 22 31 Reps. not up 12 Democrats need to gain three seats to control the Senate, if they win the presidency 2020 Senate map Democratic-held Republican-held Not up for election NH ME MT OR MN MA ID SD RI WY MI IA NE NJ IL DE WV CO VA KS KY NC TN AZ OK AR NM SC GA AL MS LA TX AK Both of Georgia’s seats are up for election 50 35 Dems. not up for election 22 31 Reps. not up for election 12 Democrats need to gain three seats to control the Senate, if they win the presidency 2020 Senate map Democratic-held Republican-held Not up for election NH ME MT OR MN MA ID SD RI WY MI IA NE NJ IL DE CO WV VA KS KY NC TN AZ OK AR NM SC GA AL MS LA TX AK Both of Georgia’s seats are now up for election 50 35 Dems. not up for election 22 31 Reps. not up for election 12 Democrats need to gain three seats to control the Senate, if they win the presidency

Voters gave Democrats control of the House in 2018, a shift of 40 seats, but Republicans gained two Senate seats in a favorable map. The 2020 map looks favorable for Republicans, too. While the party is now defending 22 seats, nearly all represent states that voted for President Trump and Mitt Romney in the past two presidential elections.

There are several Republican senators who are at risk, however, according to Cook Political Report’s ratings. The chart below shows a dot for each Senate seat up for election, positioned according to the states’ presidential voting records, from most Democratic on the left to most Republican on the right. Vertically, senators are positioned according to their ideology, using a score called DW-Nominate. The outliers show which senators’ voting positions least match their state’s ideology.

Senate seats up for election in 2020 Competitive GOP, Dem Solidly GOP, Dem How each state voted for President MORE Democratic MORE Republican Gardner (Colo.) 0.5 McConnell (Ky.) More conservative Collins (Maine) How the senator votes Isakson (Ga.) 0 Jones (Ala.) More Liberal Smith (Minn.) -0.5 D+10 EVEN R+10 R+20 Senate seats up for election in 2020 Competitive GOP, Dem Solidly GOP, Dem How each state voted for President MORE Democratic MORE Republican Perdue (Ga.) Ernst (Iowa) Roberts (Kan.) 0.5 Gardner (Colo.) McConnell (Ky.) More conservative Tillis (N.C.) Hyde- Smith (Miss.) Alexander (Tenn.) McSally (Ariz.) Collins (Maine) Isakson (Ga.) How the senator votes 0 Jones (Ala.) Peters (Mich.) More Liberal Smith (Minn.) -0.5 Udall (N.M.) D+10 EVEN R+10 R+20 Senate seats up for election in 2020 Competitive GOP, Dem Solidly GOP, Dem How each state voted for President MORE Democratic MORE Republican Perdue (Ga.) Ernst (Iowa) Roberts (Kan.) 0.5 Gardner (Colo.) McConnell (Ky.) Tillis (N.C.) More conservative Hyde- Smith (Miss.) Alexander (Tenn.) McSally (Ariz.) Collins (Maine) Isakson (Ga.) How the senator votes 0 Jones (Ala.) Peters (Mich.) More Liberal Smith (Minn.) Udall (N.M.) -0.5 D+10 EVEN R+10 R+20 Senate seats up for election in 2020 Competitive GOP, Dem Solidly GOP, Dem How each state voted for President MORE Democratic MORE Republican Perdue (Ga.) Ernst (Iowa) Roberts (Kan.) 0.5 Gardner (Colo.) McConnell (Ky.) Tillis (N.C.) Hyde- Smith (Miss.) More conservative Alexander (Tenn.) McSally (Ariz.) Collins (Maine) Isakson (Ga.) How the senator votes 0 Jones (Ala.) More Liberal Peters (Mich.) Smith (Minn.) Udall (N.M.) -0.5 D+10 EVEN R+10 R+20

Democrats are aiming to pick up at least three seats, which would be enough to give them control of the Senate if they also win the presidency. Four seats would give them control no matter how that election turns out, meaning they could block GOP legislation and appointments in the case of a second Republican term.

Sens. Susan Collins (R-Maine) and Cory Gardner (R-Colo.) represent states that rejected Trump. Republican seats in Iowa, North Carolina and Arizona will also be targets, and now two seats in Georgia. Republicans have targets of their own. Democratic Sen. Doug Jones (Ala.) is relatively centrist as far as the Senate goes, but he represents a state that heavily supported Trump and will face a tough reelection battle.

[The 10 Senate seats most likely to flip in 2020]

Of course, Senate elections are about more than a state’s voting history, and the importance of individual candidates cannot be overstated.

Georgia could help Democrats win the Senate, but only with the right person running. The state, which will now have two Senate seats on the ballot in 2020, has voted for the Republican in recent presidential contests — including for Trump by five percentage points in 2016. Democrats hope to win here, in part because of how narrowly Democrat Stacey Abrams lost the 2018 governor’s race, but they have struggled to recruit a top-tier candidate. Abrams tweeted a statement Wednesday saying that she “will not be a candidate” in either 2020 Georgia Senate race, instead focusing on her national voter protection efforts.

Competitive Senate seats in 2020 Democrat-held seats State Incumbent Filing deadline Alabama Doug Jones Nov. 8, 2019 Michigan Gary Peters April 21, 2020 Minnesota Tina Smith June 2, 2020 New Mexico OPEN March 10, 2020 Republican-held seats State Incumbent Filing deadline Arizona Martha McSally April 6, 2020 Colorado Cory Gardner March 17, 2020 Georgia David Perdue March 6, 2020 Georgia OPEN March 6, 2020 Kansas OPEN June 1, 2020 Kentucky Mitch McConnell Jan. 28, 2020 Iowa Joni Ernst March 13, 2020 Maine Susan Collins March 16, 2020 Mississippi Cindy Hyde-Smith Jan. 10, 2020 North Carolina Thom Tillis Dec. 20, 2019 Tennessee OPEN April 2, 2020

If Democrats win the presidency but lose the Senate, they will face an uphill battle in enacting their policy goals. Even if they win the Senate, though, they’d almost certainly be short of the 60-seat “supermajority” required to override a filibuster and pass key legislation. In response, many 2020 Democratic presidential candidates have proposed eliminating the filibuster. Trump has also called for eliminating the filibuster, but Senate Majority Leader Mitch McConnell (R-Ky.) has refused.

If that change occurs, the importance of winning — and holding — the Senate will be more important than ever.