Sen. John McCain (R-Ariz.) has decided not to address House Republican conservatives when they convene at their annual retreat this week, while one of his top rivals for the GOP’s 2008 White House nod will be among the headliners of the event.

Not only is McCain not attending the annual Conservative Members Retreat — which draws members of the Republican Study Committee and is sponsored by the Heritage Foundation — Republican sources said that McCain’s campaign never responded to the invitation.

The GOP’s three most prominent 2008 hopefuls — McCain, former Massachusetts Gov. Mitt Romney and former New York Mayor Rudy Giuliani — were invited to meet with the group when it gathers in Baltimore this Thursday through Saturday.

Romney will make an appearance at the retreat, while Giuliani sent his regrets and said he had a scheduling conflict. McCain’s campaign never responded to inquiries.

Other headliners of the retreat include former Speaker Newt Gingrich (R-Ga.), who is mulling a White House run, and former Sen. Phil Gramm (R-Texas), who already has signed on with McCain’s campaign.

McCain campaign spokesman Danny Diaz did not address why his boss wouldn’t attend, or why he didn’t respond to the invite, but he said the Senator enjoys many friendships on Capitol Hill and has daily opportunities to reach out to conservative lawmakers.

“While he’s not going to be able to attend this particular meeting, he’s fortunate to be able to meet every day with conservative leaders in the United States Capitol,” he said.

McCain has long had a tumultuous relationship with conservatives. His run for the GOP presidential nomination in 2000 put him squarely in the cross hairs of religious evangelicals and other movement conservatives who had lined up behind the candidacy of then-Texas Gov. George W. Bush. The South Carolina primary, dominated by the party’s most conservative wing, was especially bloody for McCain.

McCain has since mended fences with Bush and reached out to conservatives, many of whom are untrusting of the man they see as a maverick who has bucked his own party as often as he has supported it.

An aide to a prominent House conservative said that Members were more puzzled than offended at the apparent disinterest in the retreat coming from the McCain campaign.

According to sources, staff at both the RSC and Heritage reached out to the Romney, Giuliani and McCain campaigns in mid-January to schedule appearances at the annual gathering traditionally held in Baltimore.

Giuliani’s team could not attend due to an unbreakable scheduling conflict on the West Coast, and Romney accepted the invitation and is scheduled to appear Friday. More than 100 House Republicans are members of the RSC, and while attendance lists weren’t confirmed at press time Friday, about half of those Members are expected to attend.

Romney has been the most overtly active of the three in recruiting supporters in the House. A few members of the RSC already have endorsed him, including GOP Reps. Marsha Blackburn (Tenn.), Tom Feeney (Fla.), Howard McKeon (Calif.) and Dave Camp (Mich.).

One aide suggested that McCain’s campaign underestimated the importance of building early support in the House.

“The McCain campaign is missing the boat entirely and failing to grasp the importance of the momentum that adding House Members early builds,” the aide said, adding: “In light of Gov. Romney’s energetic outreach to conservatives in the House, it’s puzzling and perhaps telling about McCain’s own outreach strategy to House Members.”

A spokesman for Rep. Jeb Hensarling (R-Texas), the chairman of the RSC, said only that conservatives were looking forward to the scheduled discussions at the retreat.

“Chairman Hensarling is very pleased that Gov. Romney has agreed to speak to House conservatives and is looking forward to welcoming some of the best minds in the conservative movement to meet with Republican Study Committee members,” said Brad Dayspring. Hensarling has not yet endorsed any presidential candidate.

Romney’s appearance at the RSC event comes as his opponents are working to highlight his own ideological history, which includes a conversion from moderate to more hard-core conservative stances on several key social issues. There also is unease among conservative Christians with Romney’s Mormonism.

“Gov. Romney can go meet with House conservatives and pretend to be one of them but it’s important to remember that John McCain served in the U.S. House as a real conservative,” said one GOP official close to the McCain campaign.

Romney spokesman Kevin Madden said that the former governor has a lot in common with RSC members when it comes to both their “political and policy philosophy” and that he looks forward to talking with them about issues such as spending priorities, national security and global terrorism.

McCain does have vocal supporters within the RSC. Former RSC Chairman John Shadegg (Ariz.) and Rep. Jeff Flake (Ariz.) are both behind their Arizona colleague. Rep. Spencer Bachus (Ala.), an RSC member, signed onto McCain’s campaign as southeast co-chairman last week.