Rep. Ayanna Pressley (D-Mass.), who defeated an incumbent House Democrat in 2018, endorsed Booker on Wednesday. A handful of national progressive groups have made late endorsements: Indivisible and Democracy for America also endorsed his bid Wednesday, and MoveOn and the Working Families Party jumped on board earlier this week.

“If we're looking at a race that’s down to a couple dozen or a couple hundred votes, we think every vote will matter down the stretch,” said Lucy Solomon, the IE political director for Indivisible. They plan to send 45,000 direct mail pieces and 25,000 text messages encouraging voters and helping them manage the process of voting under guidelines set because of Covid-19.

“We really think every little bit that everyone can do will have an impact on this election, and we're seeing that up close,” Solomon said.

Indivisible's membership voted to endorse Booker — though a local group, Indivisible Kentucky, which represents Louisville, previously endorsed Mike Broihier, another candidate in the primary. Broihier's campaign sent out a release Wednesday with local members criticizing the national organization's decision to back Booker.

Much of the backing for Booker has come after he already began surging, which started as he joined protests against police brutality in his hometown of Louisville. But endorsements from Bernie Sanders and Alexandria Ocasio-Cortez last week sent a shockwave into the race, and others began to follow.

McGrath, who has mostly kept her aim set on McConnell, has massively outspent Booker on TV and is likely known by most Democratic primary voters, including those outside the population centers in Louisville and Lexington. But the influx of support has helped Booker increase his presence on the airwaves and boost his name identification.

"I'm proud our campaign for real progress and justice is catching fire, and I know we're going to have all the support we need to win on Tuesday," Booker said in a statement.

There is a limit to the benefit of the late endorsements from progressive groups, as new fundraising can only increase his TV presence so much. There’s limited time left to organize on the ground, particularly since many voters have already mailed in absentee ballots. Some progressives have expressed frustration that there wasn’t investment much earlier behind Booker and other liberal candidates in Senate primaries across the map this year.

Still, he has some in-state boosters that were already helping his bid, including the state’s two largest newspapers, Matt Jones, a popular sports radio host, and nearly two-dozen of his colleagues in the state legislature. This week, he’s added endorsements from Alison Lundergan Grimes — who challenged McConnell in 2014 — and Greg Stumbo, a longtime Kentucky Democratic figure who has served as state attorney general and state House speaker.

McGrath, a Marine veteran who lost a House bid in 2018, which was her first run for office, does not have substantial in-state endorsements from legislators or others in the political class. More than a dozen local and national unions have backed her campaign, and she has endorsements from the Democratic Senatorial Campaign Committee, VoteVets, the gun control group Giffords, Sen. Kirsten Gillibrand's group Off the Sidelines and a handful of others. Senate Minority Leader Chuck Schumer expressed confidence in McGrath earlier this week, and she remains the frontrunner in the race

“We all have the same goal here, all of us who want to defeat Mitch McConnell," McGrath said in an interview with POLITICO last week. "I don’t focus so much on endorsements. I focus on winning this primary and defeating McConnell, building a team that can defeat him because Kentucky deserves a better senator."

Still, the race is substantially closer than most Democrats predicted even weeks ago. Evan Weber, the co-founder of the climate group Sunrise Movement, which endorsed Booker early in the race, said they view his candidacy as an opportunity to “change the debate about big, bold ideas like the Green New Deal.”

“We’re thrilled the progressive movement as a whole is now lining up behind Charles, because it proves to us that leaders like him are our future,” Weber said.