So Senate Dems are set to force a high-profile vote tomorrow on the controversial Blunt amendment, which would allow employers and insurers to deny coverage for health care services they deem morally objectionable. Dems have been hammering GOPers on it for weeks, and hope to use it against them in this year’s Senate races, to peel off independents and win over women, both key swing constituencies.

One thing to watch: How many Republicans will defect and vote No?

A spokesman for Senator Susan Collins confirms to me she’s still undecided — with less than 24 hours until tomorrow’s vote.

On MSBNC just now, Senator Olympia Snowe confirmed she’ll oppose the measure, echoing the Dem argument against it: “It’s much broader than I could support.” Snowe announced yesterday that she’s retiring out of frustration over “partisanship,” which has led to some (unconfirmed) speculation among Democratic aides that the Blunt amendment is one reason why.

Senator Dean Heller’s campaign has said he hasn’t yet taken a position. Senator Lisa Murkowski appears to be undecided, but the quote she gave to the New York Times suggests she’s pretty irked over having to deal with this right now. “I don’t know where we are going with this issue,” she said. “We’ve got way too much else to be doing.”

Murkowski’s spokesperson didn’t return an email asking how she would vote tomorrow.

The stakes are high, both in individual races and nationally. If there are GOP defections, it could create complications for Senator Scott Brown, who’s facing a stiff challenge in Massachusetts, where contraception has become a major issue. Brown has sometimes joined the Maine Senators in breaking with his party. If they break this time, and he votes for the measure, as expected, it could compromise his carefully-cultivated reputation for independence, which is crucial in the Massachusetts Senate race.

For their part, Republicans insist that this issue will play in their favor in states like Nebraska, Montana, Missouri and North Dakota.

More broadly, GOP fractures on the issue could make it easier for Democrats to press birth control as a wedge; one poll found that even 50 percent of Republicans support Obama’s birth control coverage mandate. So keep an eye on GOP voting on this tomorrow. It’s key.