Sen. Joe Manchin declined on Monday to commit to advancing Democrats’ sweeping elections bill, saying he would need more assurances that his proposed changes would be adopted.

It may seem like a picayune matter given that no Republicans support the bill and the GOP is expected to filibuster the effort on Tuesday. But Democrats want to send a political message, and they need Manchin’s vote to paint a more vivid contrast with Republicans’ blockade.

The West Virginia Democrat opposes the Democrats’ sweeping elections bill but is undecided on whether to vote to start debate on it. All other Senate Democrats have co-sponsored the legislation.

Manchin has proposed a compromise bill focused on expanding early voting and ending partisan gerrymandering; it would cut some of the bill’s other elements, like publicly financed elections. He told reporters on Monday he’s still working on his compromise with fellow Democrats and that “there’d have to be an agreement to get on the substitute.”

“I hope they make some changes, agree with some changes,” Manchin said. “We put out an awful lot of changes that hopefully the country will agree with.”

Senate Majority Chuck Schumer and other Democrats have suggested that advancing the bill is the only way to adopt Manchin’s alterations. Senate Majority Whip Dick Durbin complimented Manchin’s work on Monday but also said he wasn’t sure where Manchin would come down.

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