WASHINGTON — In February, Democrats began getting anxious about the crucial Senate race in North Carolina. An unknown Republican-affiliated group was suddenly pouring millions of dollars behind the candidacy of Erica Smith, a progressive state senator who they believed had no chance of winning the seat in November.
So party leaders began spending millions of their own to bolster Cal Cunningham, a military veteran they had endorsed months earlier whom they saw as their best chance to defeat a vulnerable incumbent Republican, Senator Thom Tillis. With significant party help, Mr. Cunningham prevailed and is now in a strong position against Mr. Tillis, making him one of 14 candidates endorsed early by the Democratic Senatorial Campaign Committee who either froze out their opposition entirely or won their primaries.
Democrats took some heat for playing early favorites, including for passing over Black candidates like Ms. Smith, Charles Booker in Kentucky and Royce West in Texas, although they endorsed five candidates of color. But the strategy has paid off. With the general election field essentially set and the heart of the campaign season beginning, Democrats, riding public dissatisfaction with President Trump’s handling of the pandemic, now find themselves with a solid chance to take control of the Senate next year.
Though Democrats did not get the Republican opponent they wanted in Kansas, polls show Democratic contenders are ahead or running even with incumbents in at least seven states, with the potential to bring even more into play. They need a net gain of only three seats to take the majority should former Vice President Joseph R. Biden Jr., the presumptive nominee, win the White House — four if he does not.
Along with the pandemic, Mr. Biden’s candidacy has expanded the playing field for Democrats by depriving Republicans of some of the anti-socialist message they intended to employ had Bernie Sanders been the nominee.
But Democrats say it was their primary strategy that put them in position to take advantage of the opportunity. The nonpartisan Cook Political Report recently rated Democrats with a slight advantage in the fight for the Senate majority, and they are on defense only in Alabama, where the Democratic incumbent Doug Jones is facing long odds in a deeply conservative state. Michigan, where Republicans also hoped to pick off an incumbent, no longer appears to be in play.
“We are feeling very good,” said Senator Chuck Schumer of New York, the Democratic leader who is deeply involved in mapping Senate campaign strategy. “Republicans tried to intervene in the primaries, but we looked for the candidates who were most competitive. The bottom line is we have candidates who represent their states very well and are talking about the issues the public really cares about.”
Republicans concede they are in trouble. In a memo recently issued by Kevin McLaughlin, the executive director of the National Republican Senatorial Committee, to mark 100 days left before the election, he said, “From 30,000 feet, things look pretty bleak.”
The party averted catastrophe on Tuesday in the Kansas primary when Representative Roger Marshall, their preferred candidate, beat Kris Kobach, the former secretary of state and a polarizing conservative whom both parties judged as a likely November loser against Barbara Bollier, the Democratic recruit. Outside groups poured $10 million into the race, with Democrats trying to get Mr. Kobach on the November ballot and Republicans behind Mr. Marshall.
A Kobach victory would have solidified the perception that the Republican majority was veering to the extreme, and the party celebrated the outcome of what Senator Mitch McConnell of Kentucky, the majority leader, called a proxy war between his forces and those of Mr. Schumer.
“We slugged it out, and I won and Schumer lost,” he said.
Still, Mr. McConnell and other Republicans recognize they have their hands full in their effort to preserve the majority. They are already planning to spend millions of dollars to bolster Mr. Marshall after his tough primary, even in traditionally Republican Kansas. The majority leader and other Republicans say they always knew it would be difficult given the number of seats they had to defend, even before the pandemic knocked them on their heels.
“This was always a really challenging situation for us,” Mr. McConnell said in an interview. “We have a lot of exposure, a whole lot of close races, a massive amount of spending on both sides. It is a knife fight in an alley.”
Democrats say their advantage was established months ago as they sorted through prospective candidates and started deciding who to get behind.
“The endorsement decisions reflected a few different metrics: in-state support, fund-raising potential and the ability to put together the strongest campaign in November for example,” said Lauren Passalacqua, a spokeswoman for the Democratic Senatorial Campaign Committee.
Jessica Taylor, who analyzes Senate races for the Cook Political Report, said Democrats had found capable candidates, but argued that antipathy for Mr. Trump was the main factor driving their powerful fund-raising, and that his standing would ultimately prove determinative in the Senate fight.
“Right now, you have Democrats with the momentum, and that is driven by frustration with President Trump,” Ms. Taylor said.
Democrats suffered their first primary loss on Thursday when James Mackler, an Army veteran, was upset in the Tennessee race by Marquita Bradshaw, a longtime social justice and environmental activist who was badly outspent. But their chances of flipping the seat now held by a veteran Republican, Senator Lamar Alexander, who is retiring, were always slim.
At the top of their target list are three senators — Mr. Tillis in North Carolina, Senator Martha McSally in Arizona and Senator Cory Gardner in Colorado — who have been consistently losing to their Democratic challengers in polls. Those seats are the most likely to fall into Democratic hands.
Democratic candidates are also running neck and neck with Republican incumbents in Georgia, Iowa, Maine and Montana. A group aligned with Mr. McConnell announced that it would spend $21 million this month in Arizona, Georgia, Iowa, Montana and North Carolina, moving up a planned advertising blitz to try to stem Democratic momentum. And the battleground could expand.
New polls show the veteran Republican senator Lindsey Graham in a tight race in conservative South Carolina against Jaime Harrison, a Democrat. In Kentucky, Mr. McConnell remains ahead but Republicans have reserved significant airtime there if his race with Amy McGrath, a former Marine pilot, tightens.
Democrats sense opportunity in Texas, where the party has just announced a “seven-figure” investment against Senator John Cornyn in the hope of making him vulnerable to the Democratic pick, MJ Hegar, who received substantial financial aid from the party’s campaign committee and other advocacy groups in holding off Mr. West.
Mr. Cornyn said he anticipated the effort after Democrats invested heavily in the primary. “That wasn’t just throwaway money,” he said. “The polling is such and the environment is volatile enough that I am sure they are keeping an eye on it.”
Republicans contend that Democratic challengers have gained an upper hand in the contests not because of any inherent strength, but because of the limitations of the pandemic, which has curbed their public appearances and the typical scrutiny that accompanies them, allowing the Democrats to focus more on fund-raising. That is one reason Republican incumbents have been clamoring for debates, not the usual position of incumbents, who typically want to limit the exposure of their opponents. Republican strategists said they expect Democratic poll numbers to decline under a flood of tough advertising attacks.
“Democrats have used a global pandemic to skirt a traditional campaign schedule and avoid press questions,” said Jesse Hunt, a spokesman for the National Republican Senatorial Committee. “This weak crop of recruits has already seen voting opinion turn against them as G.O.P. ads highlight the personal scandals and disastrous policy ideas that make them unfit to represent their respective states.”
But Democrats argue the pandemic has only bolstered public yearning for a change in Senate control, a desire that could be heightened if Congress fails to deliver a recovery aid package that is currently stalled on Capitol Hill.
“People were worried about the future even before the Covid crisis, and now the crisis has exacerbated those problems,” Mr. Schumer said. “This is a time when people need some help, and Republicans are tied in knots.”