Democrats have for months tied vulnerable Republican senators to Donald Trump, hoping to damage these incumbents in an effort to win back the Senate majority. Now, with Trump all but officially planted atop the GOP ticket, Democrats are optimistic they can expand the election map and make even more races competitive in hopes of winning more than a narrow majority.

Republicans currently have a four-seat advantage in the chamber and are defending 24 seats this year to Democrats’ 10 – m the largest number of incumbent Republican senators up for re-election in 90 years, according to an analysis from Smart Politics at the University of Minnesota.

The key races are in presidential swing states like Ohio, Pennsylvania, Wisconsin, Illinois and New Hampshire, where GOP senators all face high-profile challengers.

But with Trump as the standard-bearer for his party this fall, Democrats are also eyeing races in Arizona, Arkansas, Indiana, Iowa, Missouri and North Carolina.

Most non-partisan analysts still have Republicans favored in all of those states. Jennifer Duffy, who analyzes Senate races for the Cook Political Report, said Democrats could make some of these states competitive, but expressed skepticism that they would be able to win across the board.

“Can Democrats expand the playing field? Yes. Does it mean they’re going to win all those races? The jury is very much out,” Duffy said. “Putting them in play does not necessarily equal victory, but this cycle is going to be fought on a much bigger map than we would have imagined six months ago."

Democrats are in a good position to make these more difficult states competitive, however, because of the strong candidates they recruited long before Trump’s unexpected success in the presidential primary.

In Missouri, for example, Jason Kander, the 35-year-old secretary of state and an Afghanistan War veteran, is facing first-term incumbent Sen. Roy Blunt. In Arizona, Rep. Ann Kirkpatrick, who represents a Republican-leaning district, is taking on five-term incumbent and former presidential candidate John McCain. In Arkansas, former U.S. Attorney Conner Eldridge is challenging first-term incumbent Sen. John Boozman. Indiana is an open seat, with former Democratic Rep. Baron Hill facing Rep. Todd Young in a rematch of a 2010 House race that Young won.

North Carolina may represent the biggest recruiting failure for Democrats this cycle, with state Rep. Deborah Ross challenging Sen. Richard Burr after high-profile figures such as Transportation Secretary Anthony Foxx and former Sen. Kay Hagan declined to run. But Ross has kept polling close, with Burr holding a less than five-point advantage in the RealClearPolitics average.

“I think all of a sudden, you’re starting to see how viable these races are because we’ve got good candidates in place who are going to be able to ride this wave,” said Lauren Passalacqua, a spokeswoman for the Democratic Senatorial Campaign Committee.

Most of the candidates in these races have already made Trump a central issue in their campaigns, attempting to tie him to their opponents. Last week, Eldridge, the Arkansas challenger, released a scathing ad showing multiple clips of Trump saying offensive things about women and labeling Boozman an enabler.

“He is content to say that he supports Trump and enable these comments, this hateful rhetoric,” Eldridge said in an interview. “I’m going to stand up and speak out, and this ad is about calling out some hateful, offensive rhetoric, and calling out Senator Boozman for not speaking out against it, and permitting it and enabling it through his support.”

Kirkpatrick released a similar ad in Arizona in February that called Trump “dangerous for America” and highlighted McCain’s comments that he would support him, saying that “30 years in Washing have changed John McCain.” Recent polls have shown the two candidates running neck and neck, with the longtime senator holding just a half-point lead in the RCP average.

McCain himself has acknowledged the effect Trump might have on his race. In a recording of a private fundraiser reported by Politico, he said there’s “no doubt that this may be the race of my life.” On Wednesday, McCain told reporters in Arizona he didn’t think Trump was necessarily bringing down his campaign.

"I believe that Donald Trump's candidacy puts more uncertainty into the campaign,” he said. “But the fact is, as I also have said many times, I'm running my own campaign and the people of Arizona know me."

National Republicans push back on the idea that the GOP nominee will be a drag, and insist that Hillary Clinton, the likely Democratic standard-bearer, will be a bigger drag on her party’s challengers. Andrea Bozek, the communications director at the National Republican Senatorial Committee, pointed out that Trump won more votes than Clinton in both the Arizona and Missouri presidential primaries.

“There is a reason Democrats aren’t lining up to campaign with Hillary Clinton. She is a toxic candidate whose failed leadership has put the security of our country at risk,” Bozek said. “Republicans will maintain control of the Senate because we have better-prepared candidates running better, more coherent, and more thoughtful campaigns."

For Democrats, picking off one or more of these races will be critical since the 2018 midterm map is nearly a complete opposite of this year, with Democrats defending 25 Senate seats in red states such as Montana, North Dakota and West Virginia and in battlegrounds like Ohio, Florida, Pennsylvania and Virginia.

“If Democrats want to build a durable Senate majority, they probably need to get more than just 51 or 52 seats; they probably need 54 or 55, to be honest,” said Kyle Kondik, who analyzes elections for Larry Sabato’s Crystal Ball at the University of Virginia. “So where do those seats come from?”

Some Senate Democrats agree that they will need to build a firewall against potential Republican gains two years from now.

“In the long term in 2018, the map flips on its head and is a lot tougher,” said a Democratic Senate leadership aide. “To have a stronger cushion would certainly be helpful, and we are in a good position to do that, to expand the map.”

One Senate Democratic strategist said an important pitch to donors this year will be that the Trump candidacy and favorable map are an electoral gift to the party that won’t come around again.

“We have what could potentially be a historic opportunity; we can’t leave anything on the sidelines. We need to invest in every single one of these places,” the strategist said. “Every Democrat is going to be saying that to donors from here on forward because people realize opportunities don’t necessarily come along like this."

Passalacqua, the spokeswoman for Senate Democrats’ campaign arm, however, pushed back on the notion that certain races would be difficult for Democrats two years from now, pointing out that as recently as last year, Trump’s meteoric rise would have been unthinkable.

“We are very focused on making sure we build the strongest majority we can in this cycle,” she said. “I would caution against prognosticating what might happen in 2018 given that this year Donald Trump is the presumptive Republican nominee for president.”

Duffy, the nonpartisan analyst, also cautioned against making predictions too early, though she was referring to current campaigns. She said it would become clearer which Senate races will be competitive after the parties’ national conventions this summer.

“If everything we know about politics is true, then Democrats are going to have a heck of a year in the Senate,” Duffy said. “Here’s the problem: Nothing in this cycle that we know has proven to be true."