CINCINNATI – John Kerry, telling fellow combat veterans he’s their “true brother in arms,” said yesterday that President Bush’s plan to withdraw American troops from Europe and Asia would weaken U.S. security and embolden nuclear-armed North Korea.

“This is clearly the wrong signal to send at the wrong time,” the Democratic presidential candidate said at the Veterans of Foreign Wars’ 105th annual convention, borrowing a line Mr. Bush has used against him.

Some Bush supporters and a few fellow veterans have been raising questions about Mr. Kerry’s military record, which has been a cornerstone of his presidential campaign. Senior Democrats inside and outside the campaign say they’re worried about indications, some based on polling, that the criticism might be undercutting gains Mr. Kerry made against Mr. Bush in the Democratic National Convention, with its spotlight on his combat record and military honors from the Vietnam War.

Some Democrats are urging Mr. Kerry to fight back against the criticism, which gained steam this month with a TV ad featuring fellow veterans questioning his record. Kerry campaign officials, speaking on condition of anonymity, said he may have to do so.

Addressing the same VFW crowd two days earlier, the president announced a seven- to 10-year plan to withdraw up to 70,000 American troops from Cold War bases in Europe and Asia. He accused Mr. Kerry of sending “the wrong signal” by promising to try to withdraw American troops from Iraq within six months of taking office.

Along with the economy, national security has emerged as a major theme in a campaign shadowed by war and terrorism.

The challenger hopes to convince voters he is tough enough to lead the nation against terror. The incumbent, his approval ratings hurt by the war in Iraq, is casting himself as a tested commander in chief who can guide the nation back to peace and prosperity.

Both men call the other lacking on defense, with Mr. Kerry now taking aim at Mr. Bush’s troop-shifting plans.

“Let’s be clear: The president’s vaguely stated plan does not strengthen our hand in the war against terror, and in no way relieves the strain on our overextended military personnel,” Mr. Kerry told the crowd after repeatedly reminding the veterans of his own combat experience in the Vietnam War.

“Why are we unilaterally withdrawing 12,000 troops from the Korean Peninsula at the very time we are negotiating with North Korea – a country that really has nuclear weapons?” the Massachusetts senator said. That last phrase was a reference to Iraq, which apparently did not have nuclear weapons when Mr. Bush ordered the invasion, with support from Mr. Kerry and others in Congress.

White House spokesman Scott Mc-Clellan said Mr. Kerry’s criticism of the president’s troop-shifting plan “shows a lack of understanding of the new threats we face.” He noted that Mr. Kerry expressed support on August 1 for possible redeployment of American troops throughout the world, including in Korea.

Mr. Kerry’s foreign policy adviser, Rand Beers, said Mr. Kerry would consider moving some American troops farther south on the Korean Peninsula to avoid a strike from the North. But he would not move them off the peninsula, Mr. Beers said, unless such action was part of a broader agreement with North Korea to scrap its nuclear program and reduce its conventional forces.

Yesterday, Mr. Bush received a commander-in-chief’s welcome from the VFW in this Republican-leaning city, drawing loud cheers of support. The veterans, who tend to vote more Republican than Democratic, gave Mr. Kerry a polite welcome.

Large portions of the crowd applauded each of his many promises to protect veterans’ benefits. Smaller portions vocally backed his points on Iraq and terrorism. Some veterans sat with arms folded, while others stood and clapped.

One man heckled Mr. Kerry, calling him a liar. “I’ll say it myself. He’s a liar,” said John Ranson of Fort Thomas, Ky., a 57-year-old Vietnam veteran who sat a few feet away from the heckler. He said Mr. Kerry had not supported American troops as a senator or three decades ago as a returning Vietnam veteran who protested the war.

But there was no shortage of Kerry supporters, including some who voted for Mr. Bush in 2000. Dale Hall, 49, of Franklin Park, Ill., said Mr. Bush has “got a mess overseas. I stand behind the troops, of course, but he’s not done a good job.”

Mr. Kerry tried to connect with the crowd, saying that if elected “you will have a true brother in arms” who will protect veterans’ benefits and improve the nation’s security.

He did not mention that Mr. Bush served stateside in the Texas Air National Guard, though Mr. Kerry’s allies have accused the president of using family connections to stay out of Vietnam.