At least 14 Republican members of Congress have refused to endorse or publicly support Sen. John McCain for president, and more than a dozen others declined to answer whether they back the Arizona senator.

    Many of the recalcitrant GOP members declined to detail their reasons for withholding support, but Rep. John Peterson (R-Pa.) expressed major concerns about McCain's energy policies and Rep. Walter Jones (R-N.C.) cited the Iraq war.

    A handful of other Republicans on Capitol Hill made the distinction between "endorsing" and "supporting," adding that while they have not endorsed, they do support McCain.

    In recent weeks, much of the discussion and debate about party unity has been on the Democrats' side, amid their protracted presidential primary. Yet achieving harmony is a concern on both sides of the aisle this year.

    It is not unusual for certain factions of the Democratic and Republican parties not to embrace their respective candidates for president. McCain's campaign seized on some Democrats' reticence about Sen. Barack Obama (D-Ill.), issuing a release on Tuesday that highlighted that Rep. Dan Boren (D-Okla.) is not endorsing the presumptive nominee. While some conservative Democrats have yet to endorse Obama and didn't back Sen. John Kerry (D-Mass.) in 2004, there are both centrist and conservative Republicans representing various parts of the country who are not embracing McCain.

    Republican members who have not endorsed or publicly backed McCain include Sens. Chuck Hagel (Neb.) and Jeff Sessions (Ala.) and Reps. Jones, Peterson, John Doolittle (Calif.), Randy Forbes (Va.), Wayne Gilchrest (Md.), Virgil Goode (Va.), Tim Murphy (Pa.), Ron Paul (Texas), Ted Poe (Texas), Todd Tiahrt (Kan.), Dave Weldon (Fla.) and Frank Wolf (Va.).

    Throughout his career in the House and Senate, McCain has been at odds with his party on a range of issues, including campaign finance reform, earmarks, immigration, healthcare, taxes and energy.

    Some Senate Republicans were especially irked with McCain's role in the "Gang of 14" deal on judicial nominations.

    Rep. Tom Tancredo (R-Colo.), who has been sharply critical of McCain on immigration, told The Hill in February, "I don't like McCain. I don't like him at all."

    Tancredo spokesman Mac Zimmerman said Tancredo won't endorse McCain because he fears the senator would repudiate it like he did with the formal backing of controversial pastor John Hagee.

    However, Tancredo told ABC News this week he will reluctantly vote for McCain.

    Gilchrest and Hagel, who disagree with McCain's views on Iraq, have been mum on their endorsements. Kathy Hicks, spokeswoman for Gilchrest, said, "Since he was not reelected to public office, he's keeping his thoughts private." Gilchrest lost in a Republican primary earlier this year.

    Jones, who has voted repeatedly with Democrats on Iraq, said he can't back McCain until he gets "a better explanation of the plans for Iraq and more discussion on the economy." Jones added that no one from McCain's campaign has reached out to him.

    Peterson wants answers on energy from McCain's campaign. In a recent interview, Peterson noted McCain's stance on climate change, saying, "If global warming is our top energy priority, this country has no economic future." Peterson, who didn't endorse any candidate in the GOP primary, said, "None of them made energy their top issue."

    McCain senior campaign aide Douglas Holtz-Eakin is scheduled to meet with Peterson on June 24.

    Peterson spokesman Patrick Creighton made it clear that his boss will not be pulling the lever for Obama: "I can guarantee that John Peterson is not going to publicly support or vote for Obama."

    Paul, meanwhile, is still campaigning in the presidential race, and his campaign boasts on its website that he received more than 45,000 votes on June 3 in the final three Republican primaries.

    After clinching the nomination in early March, McCain attracted less-than-expected support in the later primary states - drawing less than 80 percent of the vote in Indiana, Pennsylvania and West Virginia.

    In some ways, the lukewarm backing of some lawmakers could be a blessing, because congressional approval ratings are at an all-time low. And McCain himself has touted his independence, proudly pointing out that he has at times upset some of his Republican colleagues in Washington.

    McCain campaign spokesman Tucker Bounds said, "John McCain has strong support among Republicans and even some others in the Congress for taking principled stands. While his support is rock solid, it also shouldn't be a surprise that Sen. McCain's bold record of independence on the issues isn't appreciated by every single Republican on Capitol Hill."

    Though the vast majority of congressional Republicans said they endorse McCain's presidential bid, many supporters were hesitant to answer the question. Some of the members' spokesmen either confirmed or denied their boss's support but declined to speak for attribution about the rationale behind that member's decision.

    A few Republicans would not go so far as to say they support McCain, only confirming that they will back the Republican nominee in the general election.

    "He will support the Republican nominee, who is likely to be John McCain," said Wayne Hoffman, spokesman for Rep. Bill Sali (R-Idaho).

    Doolittle's and Tiahrt's offices did not specify their disagreements with McCain, but both lawmakers were ardent opponents of McCain's push for campaign finance reform six years ago.

    McCain has consistently mocked the so-called "Bridge to Nowhere," the infamous earmark championed by Rep. Don Young (R-Alaska). Young's office did not respond to requests for comment.

    Other members offered support for McCain, but indicated their opposition to a few of his positions.

    Rep. James Walsh (R-N.Y.), for example, publicly backs McCain, but strongly disagrees with the presumptive nominee on immigration.

    Raj Bharwani, spokesman for Rep. James Sensenbrenner Jr. (R-Wis.), said, "Since he is a Republican, he would support the Republican nominee." But, to get Sensenbrenner's endorsement, the congressman would like "to have a meeting to discuss their differences."

    Chart: GOP members not publicly backing or endorsing McCain(PDF)

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    Jackie Kucinich contributed to this article.