While Senate GOP leaders have said they would rally their Conference around the party’s presumptive nominee for president, Sen. John McCain, the Arizonan could find himself again on the outs with his party if he returns to vote on several of his signature issues, including earmark reform, climate change and immigration.

Others say the votes give McCain an opportunity to burnish his maverick credentials and woo independents. On earmarks, he is likely to be at odds with both parties, while he is likely to side with Democrats on others.

The first test of whether the GOP Conference — and particularly its leadership — are in lock step with McCain could come as early as next week, when the chamber is expected to consider a one-year moratorium on earmarks during the budget debate. The amendment, sponsored by Sen. Jim DeMint (R-S.C.) and endorsed by McCain, has garnered little support from rank-and-file Republicans, particularly the party’s “Old Bulls,” who have mounted a defense of earmarks in recent weeks.

DeMint said on Tuesday that he hoped to set a vote for the amendment during a time when McCain can be there. “Obviously, this is his baby and I consider him the champion of this,” DeMint said.

Senate Minority Leader Mitch McConnell (R-Ky.) declined to say how he will handle the vote, adding that he would wait for recommendations from an earmark task force that he has convened before making a decision. “We haven’t given up on the prospect of having some kind of consensus within our Conference on how to go forward on the earmark issue,” McConnell said.

But those recommendations are not expected until March 15 — the Saturday after the expected end of the budget debate. Republicans predicted McConnell would not take an active role in the moratorium debate, though he has been the chamber’s loudest critic of earmarking. McCain’s office declined to comment.

According to one senior GOP aide, despite the desire to unite around McCain, his positions on several big-ticket issues likely will mean the Conference will not find common ground with their nominee.

“We’re not going to back him on earmarks, or on climate change, or on immigration,” the aide said.

With Senate Majority Leader Harry Reid (D-Nev.) looking to potentially bring climate-change legislation to the floor in May — and a group of House Democrats continuing to agitate for immigration legislation this year — Republicans acknowledge that there could be instances in which McCain and the party will be on opposite sides of the political divide.

Senate Republican Conference Chairman Lamar Alexander (Tenn.), who touted McCain’s positions on fiscal responsibility, climate change and immigration as a key to his ability to reach independent voters, acknowledged that those votes will put him in conflict with the GOP. “There will be some [conflict],” Alexander said Tuesday, noting that while “I’ve supported others in this primary contest, I’m very comfortable with Sen. McCain” as the nominee.

Republicans this year find themselves in the unfamiliar position of having a presidential nominee who’s signature political issues — aside from his support for the war in Iraq — are largely outside the party’s mainstream. Both President Bush and his father had few policy stances that set them apart from the bulk of the party, while McCain, starting with his work on campaign finance reform, has made something of a career in breaking with the GOP.

A second GOP leadership aide argued that while the differences do exist, they will have little negative impact on the race. This aide noted that because the GOP is in the minority, McConnell is not responsible for setting the agenda and will not be under the kind of pressure Reid will be to bring legislation to the floor that is backed by the Democratic nominee. “I think it would be a bigger deal if we were in the majority. … I think that’s a mitigating factor,” this aide said.

Additionally, the aide noted that McCain’s willingness to stand in opposition to his party has its benefits. “He gives us entree into communities we wouldn’t normally have,” the aide said.

Alexander agreed with the notion of McCain expanding the GOP playing field. “I think his strength is he can carry our message to more than 50 percent of Americans. If we get all the Republican votes, that’s only about 30 percent of votes, and that won’t work.”