Even Mr. Schumer conceded that Ms. Abrams’s decision came as a blow, though he tried to put a positive spin on it.

“Stacey Abrams would have been a great candidate, and she’d be a great senator,” he said. “But the good news is twofold: We have other good candidates, the polling data shows that Georgia is very winnable and Stacey is going to go all out in terms of registering voters, so that we can win in 2020.”

Just who those other candidates are, however, is unclear. After Ms. Abrams bowed out, Teresa Tomlinson, a former mayor of Columbus, jumped in. Chuck Clay, a former chairman of the Georgia Republican Party, called Ms. Tomlinson “a strong and viable candidate.” But the national party has not embraced her, and Mr. Schumer is said to be looking for other contenders.

Still, many strategists say the outlook for the party is not all bad. Democrats have had strong success in Arizona, where Mark Kelly, a retired astronaut and the husband of former Representative Gabrielle Giffords, is challenging the Republican incumbent, Senator Martha McSally. Ms. McSally’s hold is tenuous; she lost to Senator Kyrsten Sinema last year but was appointed to fill the vacancy left by the death of Senator John McCain. And Mr. Kelly already has an advantage of $1 million cash on hand.

Democrats are extremely enthusiastic about the candidacy of M. J. Hegar, a motorcycle-riding, Purple Heart-winning woman who is a former Air Force helicopter pilot and who is challenging Senator John Cornyn in Texas. Ms. Hegar narrowly lost a House race in November, and some party strategists say Democrats can coalesce around Ms. Hegar now that Mr. Castro has decided not to run.

“This is a really great environment for Democrats, and in key races we have really strong candidates,” said John Anzalone, a Democratic pollster whose firm advises Senate candidates. “How can you argue with Mark Kelly and Ben Ray and Hegar?” he added, referring to Representative Ben Ray Luján, who is seeking to succeed Senator Tom Udall, Democrat of New Mexico, who is retiring.

And while the map, as Ms. Duffy said, may not be as friendly to Democrats as the numbers suggest, it does look better for them in 2020 than it did in 2018, when the party was defending 10 seats in states won by Mr. Trump. This election cycle, Democrats are defending only two seats in those states: Alabama and Michigan, where Senator Gary Peters so far has no credible Republican challenger.