* In two impassioned speeches, Kerry lays grounds for force
* Former senator's role may grow as Obama seeks Congress's support
* Top U.S. diplomat lobbies for action on Sunday talk shows (Adds background on Obama's surprise decision to seek congressional approval, Kerry, McCain, Graham quotes)
By Arshad Mohammed
WASHINGTON, Sept 1 (Reuters) - When the Senate takes up whether to back White House plans to attack Syria, there may be few more effective or passionate lobbyists for the administration than Secretary of State John Kerry, who was a member of that exclusive club for 28 years.
In two appearances that many interpreted as foreshadowing an imminent strike, Kerry last week described Syria's use of chemical weapons as "a moral obscenity" and called Syrian President Bashar al-Assad "a thug and a murderer."
According to people with knowledge of the administration's debates, Kerry has at times argued for a more muscular U.S. involvement in the conflict even as he has been its point man on searching for a diplomatic solution.
"He has been much more open-minded about potential lethal action than others in the administration," said a former U.S. official who spoke on condition of anonymity.
That approach gained prominence inside the administration although President Barack Obama's sudden decision to change tack and seek the approval of Congress has now delayed air strikes and surprised even some of his own aides, according to an account by senior administration officials.
The choice to put on hold what had appeared to be a march toward a tailored air campaign may make Kerry's role even more significant as the White House tries to convince members of Congress to support the use of force.
The son of a U.S. diplomat, Kerry came to the job of secretary of state after nearly three decades in the Senate, all of them on the Foreign Relations Committee, and a record as a Vietnam veteran.
That experience could come in handy as the White House makes its case to Congress for action on Syria in retaliation for the military's Aug. 21 attack in the suburbs of Damascus that U.S. officials say killed more than 1,400 people.
Kerry's strong remarks came on an issue where other top U.S. officials, including Obama at times, have kept a lower profile.
Obama on Saturday said he believed that military force should be used against Syria but backed away from an imminent strike to seek the approval of Congress.
"The value of John Kerry getting out there to the president is that he can speak to that audience, and he can speak to the international audience, and he can speak to the American people," said Tamara Cofman Wittes, director of the Saban Center for Middle East Policy at the Brookings Institution think tank.
In a fresh demonstration of Kerry's role as a major advocate for the president's approach, he appeared on five television talk shows on Sunday, saying sarin was used in the Aug. 21 attack and he believed Congress would approve the use of force.
The timing of any U.S. response, most likely with cruise missiles from U.S. Navy destroyers in the eastern Mediterranean, is unclear given the decision to seek congressional approval.
Some lawmakers, including Republican Senators John McCain and Lindsey Graham, said they believed Syria's use of chemical weapons required a U.S. military response but faulted Obama for not laying out a wider strategy to end the civil war.
"We cannot in good conscience support isolated military strikes in Syria that are not part of an overall strategy that can change the momentum on the battlefield, achieve the president's stated goal of Assad's removal from power, and bring an end to this conflict," they said in a statement on Saturday.
In response, Kerry said he believed they could be persuaded U.S. action would amount to more than "an isolated pinprick."
"They're good friends of mine and I respect them both. And I am convinced that we can find common ground here with them," he told ABC's "This Week" on Sunday.
That Kerry has become a primary exponent of the Obama administration's policy might come as a surprise given his penchant for making statements that have given officials at the White House and the State Department heartburn.
Speaking in June of the U.S. effort to bring the warring parties in Syria to the negotiating table, Kerry said, "This is a very difficult process, which we come to late," a statement that can be read as an implicit criticism of Obama's largely hands-off policy over the previous two years.
And earlier this summer he said the Egyptian military had been "restoring democracy" when it toppled the first freely elected president, Islamist Mohamed Mursi, in July - a widely criticized endorsement of the military rulers who brutally cracked down on Mursi's Muslim Brotherhood supporters.
Those discordant notes aside, the White House appears to see Kerry, the 2004 Democratic presidential nominee, as one of its most effective spokesmen when it comes to reaching out to the American public, Congress and the wider world.
Last week, Kerry was Obama's main advocate for using force, belying the former Massachusetts senator's reputation as a somewhat wooden speaker.
"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders, by chemical weapons is a moral obscenity," Kerry said on Monday.
'TOUGH NEW POSITION'
"On both Egypt and Syria, Kerry led the administration's articulation of a tough new position," said Jon Alterman, head of the Middle East program at the Center for Strategic and International Studies think tank in Washington.
The Syrian civil war has presented the White House with policy options that it found unpalatable at best for a U.S. president elected in part on the premise that he would get the United States out of wars in the Muslim world.
Loath to be drawn into another one, Obama has tried to keep his distance from the conflict, holding off for months before calling for Assad to go in August 2011 and rejecting advice from then-Secretary of State Hillary Clinton and CIA Director David Petraeus in 2012 that he give U.S. arms to the Syrian rebels.
Kerry, who took over from Clinton on Feb. 1, has been an advocate for doing more to help the Syrian rebels.
In Rome in late February, during his first foreign trip as secretary of state, Kerry unveiled a marked U.S. policy shift - a decision to send non-lethal assistance such as medical supplies and food directly to the rebels.
After striking a deal with Russia in May to try to revive moribund efforts to bring both sides of the conflict to the negotiating table, Kerry hinted two weeks later that the West might arm the rebels if Syria refused to come to the talks.
Obama eventually decided to take that step, but it is unclear whether any U.S. weaponry has reached the rebels.
Kerry's diplomatic initiative, however, had begun to unravel within a month when Syrian government forces in early June retook the strategic town of Qusair, which lies on a cross-border supply route with Lebanon.
A week later on June 12, in a meeting of Obama's top national security team, Kerry argued for the United States going beyond arming opposition fighters by employing air strikes, a person familiar with the talks said.
Martin Dempsey, who as chairman of the U.S. Joint Chiefs is the United States top military officer, pushed back strongly, arguing that such a mission would be complex and costly. (Editing by Alistair Bell; and Jackie Frank)