The Washington Post’s Al Kamen of In the Loop looks at the transition of Obama’s second term. (The Washington Post)

Despite all the sound and fury before and after his nomination, Chuck Hagel began his bid to lead the Pentagon as a favorite to win Senate confirmation. And he seems only to be gaining momentum.

For sure, the Republican former senator from Nebraska made his share of enemies during his two terms in the Senate, which now must consider his nomination as defense secretary, but he began with two key advantages.

One is traditional senatorial courtesy, which has almost always meant a relatively smooth confirmation process for any current or former senator chosen for a Cabinet or ambassadorial post.

More important, almost every failed nomination of the past three decades has stemmed from key defections within the president’s own party, and so far Hagel’s opposition has come almost entirely from fellow Republicans.

The prospect of significant Democratic defections grew more unlikely Tuesday when a pair of influential Senate Democrats who had been cagey about their support for Hagel came out in support of his confirmation. Many of Hagel’s critics accused him of being hostile to Israel’s interests. But on Tuesday, Sens. Charles E. Schumer (N.Y.) and Barbara Boxer (Calif.), two of the most influential Jewish Democrats, issued lengthy testimonials to Hagel’s credentials to lead the Pentagon and accepted his assurances that he would support the Obama administration’s policy of vigorously opposing Iran’s bid to obtain nuclear weapons.

“Senator Hagel could not have been more forthcoming and sincere,” Schumer said Tuesday in a 676-word statement that covered every possible controversy of Hagel’s nomination. “Based on several key assurances provided by Senator Hagel, I am currently prepared to vote for his confirmation. I encourage my Senate colleagues who have shared my previous concerns to also support him.”

“I needed comprehensive answers,” Boxer told reporters Tuesday in a conference call, explaining that she demanded that Hagel follow up their phone discussion with a letter documenting his answers on Israel and Iran as well as issues related to gay rights and female soldiers’ access to reproductive services.

Hagel’s confirmation is still not a certainty. He has only just begun the traditional process of making the rounds for face-to-face meetings with key senators, and on Tuesday the top Republican on the Armed Services Committee announced his opposition.

“Unfortunately, as I told him during our meeting [Tuesday], we are simply too philosophically opposed on the issues for me to support his nomination,” said Sen. James M. Inhofe (Okla.). Inhofe said he was concerned about looming Pentagon spending cuts: “Senator Hagel’s comments have not demonstrated that same level of concern about the pending defense cuts.”

The committee has yet to schedule a confirmation hearing, which is certain to be a lengthy session that could resemble the combative queries that former senator John D. Ashcroft (R-Mo.) faced in January 2001 after he was nominated as attorney general.

Ashcroft’s confirmation hearings, fueled by questions about racial sensitivity, lasted three days, unusually long for a former senator.

In the modern era of televised hearings, just two former senators have failed to win confirmation after being nominated. John G. Tower, a Texas Republican, lost his bid to become defense secretary in 1989 over accusations of improper drunken behavior, receiving just 47 votes. In 2009, Thomas A. Daschle, the former Democratic majority leader from South Dakota, withdrew his nomination to be secretary of health and human services after it was revealed that he had failed to pay more than $100,000 in taxes on limousine service provided to him as a member of an investment firm.

As of now, Hagel’s opponents have not leveled any accusations of questionable ethical behavior and instead have focused on his past statements and votes about Middle East policy. Conservative activists rejected his statements to Schumer and Boxer as just a “confirmation conversion,” and senior Republicans promised tough questioning.

“No closed-door White House meeting with a single Senator or a letter can erase a problematic 12-year Senate record and many troubling public statements from Sen. Hagel,” Sen. John Cornyn (Tex.), the No. 2 GOP leader, said in a statement. “Retractions of long-held positions raise serious questions about where Sen. Hagel stands on critical issues of national defense.”

Some observers have questioned whether Republicans would mount a filibuster of Hagel, requiring a 60-vote threshold, but the nominee’s deepest critics — Sens. John McCain (R-Ariz.) and Lindsey O. Graham (R-S.C.) — have in the past been reluctant to support filibusters of presidential nominees except in extraordinary circumstances. While Democrats opposed Ashcroft 12 years ago, he was confirmed with 58 votes.

That’s why defeating a nominee often comes down to defections within the president’s party. Tower’s nomination fell apart when Paul Weyrich, an influential social conservative, testified against the nominee and questioned his “moral character.”

In 2009, after similar revelations about Treasury Secretary Timothy F. Geithner’s taxes, Senate Democrats privately voiced concern about looking the other way on Daschle’s taxes. In 2005, President George W. Bush nominated a top staffer, Harriet E. Miers, to the Supreme Court, only to encounter a conservative Republican revolt.

White House officials, keenly aware of this history, moved quickly to win the backing of Schumer. While Boxer spoke by phone with Hagel last week, Schumer made an unannounced trip to the West Wing on Monday for a 90-minute huddle with the nominee, Hagel’s first face-to-face meeting with a senator.

Schumer informed Obama of his decision Monday after the meeting with Hagel, phoning the former senator Tuesday morning to formalize his support, said a Senate aide familiar with the discussion.

In his talks with Boxer and Schumer, Hagel expressed deep regret at his “Jewish lobby” remark years ago when referring to the American Israel Public Affairs Committee.

“He was very devastated by it, looking back,” Boxer said.

If Schumer and Boxer had opposed Hagel, a crucial bloc of pro-Israel Democrats might have joined them and made confirmation impossible.

“Senator Hagel realizes the situation in the Middle East has changed, with Israel in a dramatically more endangered position than it was even five years ago. His views are genuine, and reflect this new reality,” Schumer said.