* Schumer, Senate No. 3 Democrat, seen as key vote
* Boxer, California, had also held back support
* Support expected from all Senate Democrats, some Republicans
By Patricia Zengerle
WASHINGTON, Jan 15 (Reuters) - Former Republican Senator Chuck Hagel's chances of becoming the next U.S. defense secretary received a critical boost on Tuesday when two leading Senate Democrats said they had decided to vote to confirm him.
U.S. Senator Chuck Schumer of New York, the No. 3 Democrat in the Senate, said his concerns over Hagel's positions on Israel, Iran and other issues were addressed during their 90-minute meeting on Monday. He described Hagel as "forthcoming and sincere."
"Based on several key assurances provided by Senator Hagel, I am currently prepared to vote for his confirmation," said Schumer. "I encourage my Senate colleagues who have shared my previous concerns to also support him."
Critics including Republican legislators and conservative pro-Israel groups have sought to portray Hagel as anti-Israel and as someone who is not committed to preventing Iran from obtaining a nuclear weapon, accusations he strongly denies.
U.S. Senator Barbara Boxer, a senior member of the Senate Foreign Relations Committee, also said she would back Hagel.
The four-term California senator had held off announcing her support. On Tuesday, she said Hagel would have her vote -- after they had had a long conversation and after he wrote her a letter spelling out his positions on Iran, Israel, and the treatment of gays and women in the military.
"We spoke for quite a while last week and I was very pleased with that conversation," Boxer told reporters on a conference call from California.
She said she had not been aware of Schumer's announcement and that the two had made their decisions independently.
"I urge more of my colleagues to come out because from what I've seen is there seems to be a Republican push here to really go after Senator Hagel, which is really quite disturbing," Boxer said.
A decorated Vietnam veteran who split from fellow Republicans by opposing the U.S.-led war against Iraq, Hagel was nominated by President Barack Obama on Jan. 7 to replace outgoing Defense Secretary Leon Panetta.
Republicans say that are concerned that Hagel opposes sanctions and is satisfied with containing Iran, as opposed to preventing it from obtaining a nuclear weapon. They also worry that Hagel would not prevent inordinately deep Pentagon budget cuts.
Some conservative senators have already declared their intention to vote against the former Nebraska senator.
"A record of extreme views makes Hagel an unwise pick, and the president's decision points to more rancorous fights to come," Mississippi Republican Roger Wicker, a member of the Senate Armed Services Committee, wrote in Politico.
The committee will hold confirmation hearings at the end of the month or in early February, Senate aides said.
Republicans also criticized a 2006 reference by Hagel to the influence of the "Jewish lobby" in Washington. Hagel has acknowledged that he misspoke and his defenders say such concerns are overblown.
"He told me that if there's one thing in his life that he'd like to take back, it's that. And he was very devastated about it looking back," said Boxer, who served on the Senate Foreign Relations Committee with Hagel for 10 years.
Schumer and Boxer are both Jewish, and their decisions bolster the likelihood Hagel will win Senate confirmation.
Hagel has spent much of the past week calling and writing senators whose votes could be crucial for his confirmation.
Hagel is expected to garner votes from all 53 Senate Democrats and between 10 and 15 Republicans, according to one observer who has been counting votes.