WASHINGTON (CBSDC/AP) — Sen. John McCain warns President Barack Obama could face impeachment if he decides to put “boots on the ground” in Syria.

The Republican Arizona senator made the comments to KFYI Thursday following a spirited town hall in Phoenix that focused on Syria.

“The fact is [Syrian President] Bashar Assad has massacred 100,000 people. The conflict is spreading … The Russians are all in, the Iranians are all in, and it’s an unfair fight,” McCain told KFYI’s Mike Broomhead. “And no one wants American boots on the ground. Nor will there be American boots on the ground because there would be an impeachment of the president if they did that.”

McCain also slammed Obama for calling on Congress to authorize a U.S. military strike against Assad’s forces.

“The president has bungled this beyond belief,” McCain stated. “Announced that he’s going to strike and then say, ‘No, I’m going to the American Congress.’ I can’t believe how badly he’s mishandled this issue.”

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McCain repeatedly told about 150 constituents at the morning meeting that there would be no plans to send U.S. troops to retaliate for the chemical weapons attack last month near Damascus.

“I want to begin by saying to you I am unalterably opposed to having a single American boot on the ground in Syria,” McCain said. “The American people wouldn’t stand for it.

“Second of all, it would not be anything but counterproductive to do that. American blood and treasure is too precious to do that.”

McCain told KFYI that he understands why American voters are cynical about a strike on Syria.

“They are largely against any action in Syria and I understand their skepticism,” McCain said.

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McCain had planned to talk about immigration and other issues at the gathering at a public library, but Syria dominated the conversation. He was interrupted from the start by someone shouting that his response on the topic wasn’t good enough.

A few people held up signs from their seats with messages such as “Don’t bomb Syria” and “Security thru peace.”

McCain told the audience the American public soon would see irrefutable evidence that Assad was behind the deadly Aug. 21 chemical weapons attack.

“If we open the door to the use of chemical weapons and let it go unresponded to, then I think that sends a signal to other people that want to use them, that they can do so with impunity,” McCain said.

The gatherings come as President Barack Obama is requesting speedy congressional backing of a military strike in Syria.

The Obama administration blames Assad for the sarin-gas attack in a rebel-held suburb of Damascus. Obama says more than 1,400 civilians died, including at least 400 children. Other casualty estimates are lower, however, and the Syrian government denies responsibility, contending rebels fighting to topple the government are to blame.

Despite widespread condemnation of the attack from allies, few countries are likely to join the U.S. in undertaking military action if Obama moves forward with a strike.

(TM and © Copyright 2013 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2013 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)