The latest candidate to announce is the biggest name yet.

It’s early and the field isn’t totally set, but Republican U.S. Sen. Cory Gardner is facing enough competitors at this point that it’s worth looking at the Democrat fixing to knock Gardner off his Capitol Hill saddle.

If his Twitter replies are any indication of his popularity, Gardner will have an uphill battle for reelection in 2020. But even outside the realm of the Twittersphere, his seat is seen as pretty vulnerable. And we know why: That sweeping Democratic win in Colorado last fall. The results immediately put Gardner’s seat, his supporters and his party on notice.

On Thursday, Gardner’s latest and most well-known challenger emerged: former state senator and gubernatorial candidate Mike Johnston. Johnston hopes to bring the same level of energy to this race that helped him get a lot of early support during his last race (but with different results — he finished in third place in the primary).

“Colorado deserves a senator with the courage to run toward our toughest problems and the leadership to pass progressive policy on climate, health care, immigration, and gun safety,” Johnston said in a release announcing his run.

So who’s officially running against Gardner?

In addition to Johnston, these are the folks who announced and/or have filed paperwork with the Federal Elections Commission:

Activist James Blanton

Pharmacist Dustin Leitzel

Community organizer Lorena Garcia

Navy veteran Keith Pottratz

Scientist Trish Zornio

Signs point to the GOP seeing Johnston as a real-deal challenger.

Colorado Republican Party Chairman Jeff Hays was quick to issue a quip on Thursday after Johnston made his announcement to join a “crowded” Democratic field.

“After spending $8 million to be rejected by 77 percent of Colorado Democrats less than a year ago, Mike Johnston is ready to waste more cash and lose again,” Hays said in the statement. “Colorado Republicans look forward to watching Johnston and his Democratic opponents clamber over each other as they race to the left over the next 17 months.”

Johnston’s announcement came a day after Gardner told the Independent Review he would be supporting President Trump’s 2020 reelection bid. Trump is not a popular guy in Colorado, which he lost to Hillary Clinton in 2016.

“I don’t think a candidate is going to galvanize a Democratic electorate just by being anti-Trump because that’s an expectation,” progressive policy consultant JoyAnn Ruscha said. “It’s a low bar. … That’s not going to move people.”

What will move people? Showing people what you support, not just what you’re opposing. That means laying out a clear message on where you stand on things like healthcare, which Ruscha believes will be a big issue in this race.

Blueprint Strategies co-founder and Republican campaign consultant Cinamon Watson called Johnston “a guy who weathered a really spirited primary.” She questioned whether voters might have some fatigue about having him on another ballot.

“Aren’t you tired, dude? Don’t you want a break?” Watson said on Thursday.

Watson said Gardner is no stranger to tough races.

“He took out an incumbent, a relatively popular one, in both the U.S. House and Senate,” Watson said, referencing Gardner’s defeat of Democrats Betsy Markey in 2010 and Mark Udall in 2014. “Cory Gardner is a very — and I don’t use this term lightly — a joyful campaigner.”

Watson said the senate race will likely end up becoming a pretty expensive race, similar to last year’s gubernatorial showdown.

“Democrats are looking down the barrel of a pretty long, tough primary,” Watson said. She doesn’t think Colorado’s political leanings have changed despite the big losses by Republicans; she pointed out the failure of several state’s ballot issues that showed voters have more nuanced opinions. “It’s a purple state, so a lot can happen.”

Ruscha sees it differently.

“It’s not really a purple state anymore,” Ruscha said, “I think we’ve settled that.”

While Ruscha doesn’t believe we’ll see an 11-point win like Colorado’s governor’s race, she still said given the current political climate, the seat is Democrats to lose.

So who might run against Gardner?

These are folks who have been mentioned for a possible run, but haven’t announced yet:

Former House Speaker Crisanta Duran

Former Gov. John Hickenlooper

U.S. Rep. Ed Perlmutter

Former House Speaker Andrew Romanoff

Romanoff’s announcement reportedly could come soon, while Hickenlooper’s out there serving Midwesterners fancy beer, so he seems deadset on the presidential course. Perlmutter hasn’t said much, but he was included in Denver Post story about potential challengers for Gardner.

Democratic consultant Steve Welchert said the largest share of big local and national Democrats are likely going to take a wait and see approach for Senate candidates.

Does he have an inkling on who they’re waiting for?

“I think a lot of them are waiting for Hickenlooper,” Welchert said.

Notably, both Duran and Zornio took to Twitter on Thursday to criticize Gardner for announcing his support for Trump. It’s only the second time Duran’s ever tweeted directly at the senator.

“There will be no lack of momentum to defeat them both in 2020 & ensure we have a Senator that respects the will of the people of #Colorado,” Duran said in her tweet.