This is probably the most significant movement in the debate over gun reform that we’ve seen since Obama first unveiled his package of proposals last December.

Later today, Senator Patrick Leahy, the chairman of the Judiciary Committee, will roll out a compromise proposal — with bipartisan support — on a key piece of Obama’s gun control agenda: The measure designed to crack down on gun trafficking and so-called “straw purchasers.”

Senate aides familiar with the talks tell me that Senator Susan Collins will support the measure today — a real breakthrough in terms of getting Republican support for significant legislative action on guns. Collins’ office didn’t immediately return an email requesting comment. The other Senators supporting this measure are Kirsten Gillibrand, who has had a leading role in pushing it, Mark Kirk (a second Republican attaching his name to the bill), Dick Durbin, and Leahy. The bill will be marked up in committee later this week.

The new legislation will blend two previously existing proposals — one championed by Leahy and Durbin; the other by Gillibrand and Kirk — into one bill. As one aide put it to me:  “This legislation will for the first time make gun trafficking a federal crime in order to provide tools to law enforcement to get illegal guns off the streets and away from criminal networks and street gangs. Currently, there is no federal law that defines gun trafficking as a crime.” The measure will also stiffen penalties for “straw purchasers” who knowingly buy guns for those who are not supposed to have them — a practice that many law enforcement groups and other experts believe contributes to gun violence.

The bipartisan support for the new legislation is significant: It shows there is an a persistent appetite in both parties for Congressional action in the wake of the Newtown slayings. This idea also has Republican support in the House: GOP Rep. Scott Rigell recently rolled out his own version of it, and he is said to be trying to line up GOP backing in that chamber. He recently called on the House GOP leadership to allow a vote on this proposal.

What remains to be seen is how this plays into the current bipartisan discussions that are underway over expanding background checks. The talks — which center around senators Tom Coburn, Joe Manchin, Chuck Schumer, and Kirk — have hit a snag over details involving how, or whether, records on sales processed through the expanded background check system should be kept. However, there remains room for common ground around a somewhat weakened proposal that would still nonetheless be better than what we have now. And as always, on each of these no-brainer proposals, keep an eye on how Republicans in suburban districts and districts that went for Obama react.

One question is whether the appearance of bipartisan movement on the anti-trafficking piece will encourage movement on background checks. The anti-trafficking piece is itself significant — it may prove hard for Republicans to oppose, because it is virtually impossible to demagogue this proposal as an attack on “gun rights.” But if bipartisan agreement develops around both of the proposals, that will be even more important, representing serious movement in the direction of passing two-thirds of Obama’s gun control agenda. As Doyle McManus rightly details, getting these two measures alone — without an assault weapons ban — would amount to a significant victory.

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UPDATE: Senator Collins spokesman Kevin Kelley confirms to me that she will announce the legislation today with Senator Leahy.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.