While most of the media’s attention will be riveted on an arcane, but historic, procedural fight in the Senate, an only slightly less bizarre floor debate will occur today on the other side of the Capitol.[IMGCAP(1)]

The House will take the rare step today of voting on legislation that the Republican leadership would prefer not to have scheduled at all: an embryonic stem-cell

research bill sponsored by Rep. Mike Castle (R-Del.). The measure is expected to pass with a combination of Republican and Democratic votes, and would thus represent a setback for the Republican Party brass. Sort of.

First of all, by dealing with the Castle bill now, GOP leaders are getting a contentious vote out of the way 18 months before the next election. And in exchange for a floor vote and a pledge by the leadership not to whip against their bill, Castle and Rep. Mark Kirk (R-Ill.) made some concessions of their own, thus ensuring that it will be harder for Democrats to bring the measure up again during campaign season.

“The agreement is we allow the moderates an up-or-down vote on this bill, and in return they’ve agreed to stave off Democrats’ attempts to attach this piece of legislation to other bills,” said a Republican leadership aide.

Second, today’s suspension calendar features a competing measure on umbilical stem cells authored by Rep. Chris Smith (R-N.J.) that will likely draw the support of a broad cross-section of the GOP Conference, including the entire leadership — an unusual partnership given that those same leaders took away Smith’s chairmanship of the Veterans’ Affairs Committee earlier this year.

And third, President Bush has vowed to veto any bill that allows federal funding for embryonic stem-cell research, meaning that Castle’s bill would become law only if it can garner two-thirds support in both chambers.

Still, the unusual politics behind the stem-cell issue has put Republican leaders in unfamiliar positions.

Though he opposes the Castle bill, Speaker Dennis Hastert (R-Ill.) has made good on his promise to grant it a floor vote. Last week, he was forced to step in and calm intraparty tensions after the moderate Republican Main Street Partnership sponsored polls on the stem-cell issue in several GOP-held districts.

The polling prompted an argument that nearly produced blows on the House floor between Kirk, one of the GOP Conference’s smallest members, and Rep. Rick Renzi (Ariz.), one of the biggest. The dispute boiled over at a full Conference meeting, and Hastert was forced to tell both sides to cool it.

Majority Leader Tom DeLay (R-Texas), meanwhile, is a staunch opponent of the Castle bill. Though he is bound by Hastert’s promise not to whip in opposition to the measure, DeLay does plan to organize and lead the floor debate against it.

Majority Whip Roy Blunt (R-Mo.) is also opposed to the measure but will not be mobilizing his floor operation against it, while Republican Conference Chairwoman Deborah Pryce (Ohio) will break ranks with her fellow leaders to vote for the Castle bill while also supporting the Smith measure on umbilical cord stem cells.

In that regard, Pryce will be following the same playbook as most House Democrats.

“The vast majority of Democrats will support the Castle bill,” said a Democratic leadership aide. “Even some pro-life Democrats will support it because of the way it’s structured. It’s viewed as a moderate bill and a good compromise.”

At the same time, the aide said most Democrats will also support the umbilical cord bill, which is being co-sponsored by Rep. Artur Davis (D-Ala.).

As on the Republican side, Democratic leaders do not plan to whip on the Castle bill and have instead deemed it a “vote of conscience” for their Members.

But even if the leadership doesn’t twist arms today, plenty of persuading will happen on the floor at the rank-and-file level. Castle, Kirk and Rep. Diana DeGette (D-Colo.) have taken the lead in rounding up support for their measure.

On the other side of the issue, GOP Reps. Mike Pence (Ind.), Dave Weldon (Fla.), Joe Pitts (Pa.) and Renzi have been particularly active. Rep. Scott Garrett (N.J.) is running the whip effort against the Castle bill on behalf of the conservative Republican Study Committee, which has held a series of media events in the last week in order to highlight the alleged flaws in the Castle bill and the promise offered by other types of stem-cell research.

Supporters of the Smith umbilical cord bill hope that it will give cover to conservatives who oppose federal funding for embryonic research but are wary of getting on the wrong political side of the issue. They see the Smith measure as a vehicle to make the case that they are not opposed to all stem-cell research.

With the Senate now tying itself in knots over the filibuster rule, it is unclear when, if ever, the Castle bill might be scheduled in that chamber. As in the House, a good deal of support exists in the Senate for federal funding of embryonic research, but there may not be enough to override Bush’s promised veto.

Four and a half years into his presidency, Bush has yet to veto a bill. Opponents of the Castle bill hope that the veto threat might provide added incentive for some Republicans to vote against the measure so it does not get the two-thirds support necessary to override a veto.

“The wavering Members will keep that in mind,” a Republican leadership aide predicted.