"With her impressive record of achievement and her unique life experience, Jackie will bring to Washington all the heart and soul, muscle and grit that makes for an effective new member of the House," Lantos, D-San Mateo, said in a written statement.
Lantos, the 27-year veteran of the House of Representatives, announced two weeks ago he has cancer of the esophagus and will not seek a 15th term.
On Sunday, Speier, 57, held a news conference to announceshe would run for the 12th District seat, which represents San Mateo County and part of San Francisco. The former state senator and assemblywoman had reportedly considered challenging fellow Democrat Lantos before he discovered his illness.
Lantos, 79, said Speier has been "a first-class public servant who has made the community's most pressing priorities her own" since she started in politics as an aide to Rep. Leo Ryan. He was shot dead outside Jonestown, Guyana, in 1978 after investigating Jim Jones' Peoples Temple cult.
Speier, who also served on the San Mateo County Board of Supervisors for six years, said in a statement that she is "honored and humbled" to have Lantos' endorsement. She called him "the conscience, not only of the Congress, but of all Americans on human rights issues around the world for almost three decades.
"Congressman Lantos' endorsement is by far and away the most important to me, both personally and professionally," Speier said, adding, "We continue to pray for Tom and Annette during this trying time."
In an August interview with MediaNews, Lantos criticized Speier, saying she lacked foreign policy experience and that he was confident he would be re-elected even if she ran against him.
But Lantos' spokeswoman, Lynne Weil, said Wednesday, "He would not endorse her if he didn't have full confidence in her abilities."
His statement called Speier "our best candidate for the United States Congress."
Other local politicians have thrown their support behind Speier, including U.S. Reps. Anna Eshoo and Mike Thompson, state Assemblyman Ira Ruskin and San Francisco Mayor Gavin Newsom.
No other big-name candidates have entered the race, though Yul Kwon, a former contestant on the CBS show "Survivor" and a San Mateo native, is reportedly considering a run.
Rep. Mike Honda, D-Campbell, who had worked with Kwon to promote bone marrow donation, said the management consultant recently asked him for his thoughts and advice about running for Lantos' seat.
"I think he'd be an excellent candidate," Honda said. But Honda pointed out that going up against Speier, who is widely considered the favorite, would be an enormous challenge.
The only other candidate to enter the race for the June Democratic primary is Robert Barrows, a San Mateo resident who received 7 percent of the vote in the 2006 primary.
Another possible candidate, state Sen. Leland Yee, D-San Francisco, announced earlier this week he would not run.
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