While Republican Chuck Hagel appears likely to win confirmation to lead the Pentagon, many senators in both parties are still holding back their formal endorsements or opposition.

Officially, many say they are waiting for the former Nebraska senator’s nomination hearing Thursday before the Senate Armed Services Committee.

Each side has multiple concerns, and some are not directly related to what Hagel represents personally.

Doubts about his views on Israel and gay rights have proved complicated for Democrats, who normally rush to bless Cabinet picks from a president in their own party. And Republicans have their own thorny calculations, particularly since many of them served with Hagel in the Senate.

Congressional aides from both parties have drawn the conclusion that because Hagel is likely to be confirmed, it would be better not to alienate a new Defense secretary.

“This is very tricky,” one GOP aide noted. “We will have to work with him.”

Republican Questions

Hagel has won the backing of at least one Republican senator, Thad Cochran of Mississippi, the ranking member on the Defense Appropriations Subcommittee, who said Tuesday that he plans to vote for confirmation.

The one GOP senator who is openly threatening to hold up the nomination has been careful to say that any filibuster would be related to the Obama administration’s forthrightness with Congress on last year’s fatal attack on a U.S. diplomatic facility in Benghazi, Libya.

Sen. Lindsey Graham, R-S.C., told Fox News Monday night that he was “absolutely” ready to block Hagel until Defense Secretary Leon E. Panetta testified on the Benghazi attack. Carl Levin, D-Mich., the chairman of the Senate Armed Services panel, shrugged off the threat, saying Tuesday that a Panetta hearing was in the works.

Graham has also said he is worried about Hagel’s views on Iran and Israel.

Many other Republicans, including Rob Portman of Ohio, Johnny Isakson of Georgia and Lamar Alexander of Tennessee, said Tuesday that they remain undecided on Hagel.

Sen. Susan Collins of Maine recently met with Hagel for 90 minutes. The conversation, she said, ran the gamut from personnel issues like military suicides and the continued implementation of the “don’t ask, don’t tell” repeal to the declining defense budget. Hagel, she said, assured her that the looming across-the-board sequester cuts would be catastrophic for the Defense Department, as Panetta has warned.

“That seems to represent an evolution in his comments where he said the Pentagon was bloated,” Collins said, pointing to one of the arguments Republicans have made against Hagel being the next Pentagon chief.

But Collins, who sat on the Senate Armed Services Committee until the beginning of this Congress, said she will withhold judgment until after Hagel’s confirmation hearing this week. “I want to see what he says before the Armed Services Committee,” she said, adding that she is particularly interested in Hagel’s responses before the panel on Iran sanctions.

Alexander said he plans to meet with Hagel next week, and is withholding a decision until he gets more details from the nominee. Portman, another former Senate Armed Services Committee member, said he continues to have concerns about Hagel’s nomination and is interested to hear what his colleagues who served with Hagel think.

Working on Democrats

More broadly, while Hagel has drawn heavy fire from some Republicans, particularly when it comes to his views on Iran and the U.S. nuclear arsenal, he has had to spend an unusual amount of time trying to appease Democrats in the Senate, often through multiple phone calls and meetings.

Hagel has worked to explain a past statement where he called Israeli lobbyists the “Jewish lobby,” votes in which he opposed sanctions against Iran and his calls for direct talks with that nation. Hagel has made negative comments about gays for which he has apologized. He has voted on several occasions to deny female service members the right to abortions in military facilities.

These issues, for some Democrats, have stood in stark contrast with the policies of the Obama administration. First, the administration does not speak directly with Iran and has supported the toughest unilateral sanctions to date, although, as a candidate in 2008, President Barack Obama also called for dialogue with rogue leaders. The president also lifted restrictions on gays and lesbians serving openly in the military and recently signed into law changes enabling women to receive abortions in military facilities.

At a time of heady social changes in the military, Democrats have noted the apparent incongruity between the positions taken by Hagel in the past and the course set under the Obama administration.

Democrat Tammy Baldwin of Wisconsin, the first openly gay senator, had expressed concerns about comments Hagel made in 1998 about an ambassador nominee he called “openly, aggressively gay.” She said Tuesday that she had a “very productive meeting” with Hagel last week covering a wide range of issues.

Kirsten Gillibrand, D-N.Y., a member of the Senate Armed Services Committee, has met with and spoken to Hagel several times. Gillibrand is a strong advocate for the rights of women, gays and lesbians. She also represents a state with strong ties to Israel.

“I was able to express my concerns with some of his past statements and votes, with regard to Iran and Israel, with regard to women’s rights and LGBT rights, and we had a significant conversation on each of those topics,” she said. “I felt his answers were sincere and he was determined to enforce the policies of the Obama administration. He assured me he would be a leader on women’s rights and gay rights and implement the repeal of ‘don’t ask don’t tell’ and stopping the degree of violence against women in the military.”

But Gillibrand stopped short of endorsing Hagel, as have a number of lawmakers in her conference.

Senate Majority Whip Richard J. Durbin, D-Ill., said Monday that his tallies to date showed “no ‘no’ votes” for Hagel among Democrats, whose caucus holds a 55-seat majority.

Sen. Tim Kaine, D-Va., said he believed the endorsement of Hagel by Sens. Charles E. Schumer, D-N.Y., the conference chairman, and Barbara Boxer, D-Calif., both staunch Israel supporters, put to rest many Democratic concerns about Hagel’s positions on Israel and Iran.

“I though that was the case as well, and I have not had anyone in the caucus say anything to me expressing concern,” Kaine said.

Nonetheless, Kaine, too, is withholding his endorsement.

“I’m not going to declare in advance of the hearing because it kind of makes the hearing process less relevant,” he said. “But I had a very good discussion with Sen. Hagel, first by phone and then in person, talked about a series of issues.”

One senior Democratic aide said that while some Democratic lawmakers have genuine doubts, many are willing to place their faith in the president’s positions, also recognizing the need not to foul relations with Hagel.

“I am leaning toward supporting Senator Hagel,” said Mary L. Landrieu, D-La. “I think he was an excellent senator: knowledgeable, dedicated, appropriate temperament, willing to find common ground, just the kind of leadership we need in Washington. But I’m not going to make a final decision until I go through the interview process and give my constituents time to give me their views.”

Emily Cadei and Meredith Shiner contributed to this report.