Sen. Claire McCaskill is expected to align with Sen. Tom Coburn (R-Okla.) as soon as this week to push for a mandatory ban on earmarks through fiscal 2013, warning that Democratic opposition to a ban could hurt the party with voters.

The Missouri Democrat is the leading Senate Democratic voice in favor of a ban on earmarks. Until Sen. Mark Udall (D-Colo.) endorsed the idea Monday, McCaskill was the only Senate Democrat to back a moratorium. Senate Minority Leader Mitch McConnell (Ky.) announced Monday that he was changing his position on the issue and would support a moratorium in his Conference. Republicans are set to vote on a resolution banning earmarks later Tuesday.

The Republican resolution is voluntary; the Coburn-McCaskill proposal would be mandatory.

According to GOP sources, the bipartisan duo is likely to try to attach the ban to a food safety measure the Senate will take up Wednesday.

When asked if Democratic opposition to an earmark ban demonstrates they are out of touch with the public, McCaskill responded, “Yes.”

Democrats are expected to discuss the issue during their caucus lunch Tuesday afternoon. McCaskill said that while she has not yet talked to junior members in the Conference about the proposal, she is hoping they will join her: “Mark surprised me yesterday, so I’m hopeful for more surprises today.”