SAUL LOEB via Getty Images Tim Kaine (right), appearing here at a Clinton campaign event, is widely considered the leading candidate for the Democratic vice-presidential nominee.

ST. LOUIS ― With presumptive Democratic presidential nominee Hillary Clinton set to announce her running mate in the coming week, many progressive activists and lawmakers fret that her choice may undo much of the work her campaign has done to court the party’s left wing.

At this weekend’s annual Netroots Nation conference, the year’s largest gathering of progressive activists, a number of attendees expressed concern that a more moderate choice ― such as Sen. Tim Kaine (D-Va.), who is widely considered the leading candidate ― would suppress turnout and engagement.

Asked about a Clinton/Kaine ticket, Rep. Alan Grayson (D-Fla.), who is running in his state’s Democratic Senate primary, let out a disdainful chuckle. “What can I say? I worry he’s well to the right of the mainstream Democratic Party,” he said.

Adam Green, the co-founder of the Progressive Change Campaign Committee, a prominent liberal advocacy group, contended that a Democratic ticket with a moderate vice presidential candidate could hinder Clinton’s campaign operation. “An energizing vice presidential pick will get millions of people to not just vote, but volunteer and give money,” Green said, “as opposed to merely showing up on election day.”

Many Netroots attendees expressed hope that Clinton would name Sen. Elizabeth Warren (D-Mass.), citing Warren’s stance on a number of issues and her popularity.

“I think it signals the direction of the party,” said Senator Jeff Merkley (D-Ore.), who was the only member of the Senate to endorse Bernie Sanders’ (I-Vt.) presidential campaign, of a more liberal running mate. “It would very much capture a lot of momentum for progressive issues.” Merkley, who many attendees floated as a potential running mate, added that he would also be pleased if Sen. Sherrod Brown (D-Ohio) or Sanders were tapped as Clinton’s number two.

Merkley declined to comment on anxiety about Kaine, only to say that the Virginia senator is “very capable.”

“I think most progressives would love to see Elizabeth Warren,” said Grayson. “There’s an obvious warmth that people feel toward Elizabeth Warren that isn’t duplicated by any other Democratic figure. People feel that they can believe in her, which is very important in motivating our folks to vote.”

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The Clinton campaign, for its part, has made a concerted effort to court the party’s progressive wing, both during the primary and after Sanders dropped out earlier this week. The campaign had multiple representatives at Netroots, appearing on panels dedicated to advancing progressive agendas. Many attendees spoke favorably of Clinton’s recent expansion of her proposal to reduce college debt to provide free tuition to working-class families. And, in a video message aired at Netroots, Clinton said she would introduce a constitutional amendment early in her administration to undo the Citizens United ruling that opened the door for today’s anything-goes campaign finance regime.

“I think the campaign has responded responsibly,” Grayson said.

Kaine, for his part, has been emphasizing the progressive aspects of his record on some issues of particular import to the party’s left flank. In an interview with CNN, Kaine said he is “a strong supporter of Roe v. Wade,” likely a reference to his past remarks that he personally opposes abortion. Many abortion rights proponents have expressed concern over his stance on the issue.

Despite the rapprochement between the Clinton campaign and progressive activists, many in the party’s base, in particular Sanders supporters, remains deeply skeptical of Clinton’s move to the left and her vice presidential pick.

In a press release, a group of Sanders delegates released the findings of a survey of 250 Sanders-supporting delegates, saying a majority would “Nonviolently and emphatically [protest] in the convention hall during Clinton’s acceptance speech” if Clinton picked a running mate that didn’t support a liberal enough agenda.

“Can any Democratic presidential candidate afford to do without solid support from this base in a general election?” Karen Bernal, a delegate from Sacramento, said in the group’s statement. “That is the question Secretary Clinton and her advisers should think long and hard about.”