Nearly a year before the 2020 elections, the Senate battlefield continues to take shape. Even though the executive and legislative branches are different but coequal branches of government, their fates are electorally tied together this cycle.

Democrats’ chances of controlling the Senate next Congress dramatically increase with a White House victory because the vice president would act as a tiebreaker, lowering the number of GOP seats the party has to take over. And the party that controls the Senate will determine the success and effectiveness of a new Democratic president or President Donald Trump in his second term.

[Why Georgia will be the wildest ride in politics heading into 2020]

The Democratic path to control is a little easier with North Carolina, where GOP Sen. Thom Tillis faces a competitive primary, and the GOP Senate nominee and Trump will face competitive general elections. The Inside Elections rating in that race has changed from Tilt Republican to Toss-up. Tillis joins Colorado’s Cory Gardner and Arizona’s Martha McSally as the most vulnerable Republican senators in the country.

Considering a likely loss by Sen. Doug Jones in Alabama, Democrats would need to take over at least four GOP seats. Even though Georgia is consistently mentioned as an emerging swing state, Democrats are struggling to find top-tier candidates against Sen. David Perdue or against the upcoming appointed senator who will replace Johnny Isakson. Both races could develop, particularly if Trump’s reelection prospects tank. But, for now, Democrats are distinct longshots. The Inside Elections rating in both Georgia races moves from from Lean Republican to Likely Republican.

And in Iowa, Trump’s prospects are unclear. He won the state by a significant margin over Hillary Clinton in 2016, but his trade policies could become a liability over the next year. GOP Sen. Joni Ernst hasn’t made a specific mistake, but she could be so closely tied to Trump, particularly during the impeachment process, that their fates might be connected. The Inside Elections rating changes from Likely Republican to Lean Republican.

Democrats probably need to win at least one race out of Iowa, Georgia, Maine, Texas, and Kansas for control of the Senate. Individually, each of those races has its challenges. But collectively, odds are good that Democrats can win (or Republicans can find a way to lose) less than 20 percent of that batch.

Updated analysis on all 35 Senate races is available in the Oct. 11 issue of Inside Elections.