Everyone in the Senate is getting along just fine — for now. Next year, however, the traditional civility will be tested with control of the chamber up for grabs.

Split-party Senate delegations, particularly those where the relationship between a state’s two Senators is testy to nominal at best, could face the added pressure of a heated 2012 election season as the Senator not on the ballot hits the campaign trail and works behind the scenes to defeat the one who is. This election-year dynamic is typical, but it could hold more significance in a chamber where Democrats’ four-seat majority is in jeopardy.

Interviews last week with a half-dozen Senators positioned to be on the attacking end confirmed that they intend to be active in supporting their party’s nominee against their state colleague. Members were hesitant to telegraph plans at this early stage of the cycle and said much depends on who wins their party’s nomination and the flow of the campaign as it unfolds. Still, none argued that Senate civility would preclude becoming involved where appropriate.

“I’ll support the nominee and I’ll do that actively,” Sen. Roy Blunt (R) said when asked what role he will play in trying to defeat fellow Missouri Sen. Claire McCaskill (D). “At the same time, I’m going to do my best to work with Sen. McCaskill on the things that we can agree on. My view is that 2011 is best spent finding the things we can agree on — and there will be a number of them — and 2012 being very straightforward about the things we don’t agree on. There will be a number of those things as well. We’ll have a good nominee. Missouri’s a competitive state, and I’ll be helping that nominee.”

Blunt and McCaskill have been players in Missouri politics for several years and were familiar with each other before Blunt landed in the Senate this year. Though not particularly close, the Senators described their relationship as cordial and professionally productive. McCaskill, who lost a 2004 gubernatorial bid to Blunt’s son, was not surprised to hear that her Senate colleague would be working against her in 2012.

“I don’t know how active Roy will be. I imagine in the election season I would expect to see him campaigning for a Republican nominee,” McCaskill said. “I think it would be naive of me to think that he wouldn’t be campaigning for the Republican nominee.”

At least eight targeted Senate races feature Members in split-party delegations. Most in this group are Democrats, and most are running in either decidedly conservative states or swing states. But a few are Republicans, including Sen. Scott Brown (Mass.), who serves with Sen. John Kerry (D), and Sen. John Ensign, although the Nevadan is not assured of emerging from his state’s primary. He serves with Majority Leader Harry Reid (D).

Democrats expect Kerry to engage in the race against Brown and be helpful to the eventual Democratic nominee. But Massachusetts’ senior Senator declined to comment, saying he “can’t do any politics right now.”

Brown reacted similarly in discussing whether Kerry’s expected role in helping to defeat him would affect their interaction. “I’m so focused on 2011. John and I have a good relationship, and I’m trusting it will stay that way,” he said.

Reid and Ensign observe a nonaggression pact that prevents them from hurling negative rhetoric at each other in public. It does not prevent them from campaigning on behalf of each other’s opponents or participating in other political activities, such as fundraising and strategizing. It remains unclear whether Reid will be more active on behalf of the Democratic nominee in the event that Ensign is beaten in the GOP primary by Rep. Dean Heller, who is preparing to jump into the Senate race.

A Democratic Senate aide said Reid would continue to honor his pact with Ensign but would ensure that the Nevada Democratic Party “has the resources it needs to compete in all races.” The effective voter-turnout infrastructure that Reid assembled for his 2010 race is still in place and ready to be utilized for in 2012.

“He campaigned for Jack Carter in my last election; I campaigned for Sharron Angle,” Ensign said, explaining his nonaggression pact with Reid. “We just don’t say anything negative. Our agreement is just not to criticize each other.”

The Democrats potentially under fire from home-state Republican colleagues include Sen. Sherrod Brown, who represents Ohio with Sen. Rob Portman; Sen. Bob Casey, who serves Pennsylvania with Sen. Pat Toomey; Sen. Herb Kohl, who represents Wisconsin with Sen. Ron Johnson; Sen. Ben Nelson, who serves Nebraska with Sen. Mike Johanns; and Sen. Bill Nelson, who represents Florida with Sen. Marco Rubio.

Brown worked hard on behalf of Portman’s Democratic opponent in the 2010 campaign, and their relationship is still rocky. Portman confirmed in a brief interview that he would support Brown’s GOP opponent and hinted that his support could be extensive.

Rubio has a key fundraising role with the National Republican Senatorial Committee and is expected to raise money for the state party and campaign for Nelson’s opponent and the entire Florida GOP ticket in 2012, according to an aide to the Sunshine State freshman.

The Nebraska delegation could generate a particular amount of election-year friction. Johanns and Nelson, both former governors and not always on the friendliest of terms, are playing nice right now. Nelson said the two are cordial during the Nebraska Congressional delegation’s weekly Wednesday morning breakfast, and Johanns said their offices work together well despite numerous policy differences, including on the health care reform law that caused Nelson so much political damage with Cornhusker State voters.

But that could change in 2012. Johanns said he will support the Republican nominee, although he made it clear that he will “definitely not be involved in the [GOP] primary” and demurred when asked to speculate as to the extent of his involvement in the race. Nelson indicated he would be unhappy if Johanns were to be too aggressive and overt.

“I would hope it wouldn’t be contentious,” Nelson said. “I didn’t really participate in his election two years ago.”

A Washington, D.C., lobbyist with relationships on both sides of the aisle said the Nebraska race and others are likely to create a contentious political atmosphere in the Senate, as has increasingly been the case over the years. Particularly if control of the Senate hangs in the balance, leadership will put enormous pressure on Senators to aide challengers.

“Members are cautious,” this lobbyist said. “But if they smell blood in the water, forget it.”