The top Republican on the US Senate Armed Services Committee said Nato and Afghan troops would prevail in the war if they could succeed in securing and bolstering governance in the Taliban stronghold of Kandahar.
But Senator John McCain, who visited Afghanistan's largest city in the south with two other US politicians, warned of tough fighting ahead and predicted that casualties would rise in the short-term.
"The Taliban know that Kandahar is the key to success or failure," Mr McCain said at the airport in Kabul.
"So what happens in this operation will have a great effect on the outcome of this conflict. But I am convinced we can succeed and will succeed, and Kandahar is obviously the key area. And if succeed there, we will succeed in the rest of this struggle."
McCain, a Republican from Arizona, also reiterated his opposition to President Barack Obama's plan to begin withdrawing troops from Afghanistan beginning in July 2011.
Mr Obama has said that large numbers of troops would not be pulled out if conditions did not allow, but that caveat has often got lost in the discussion over the length of US commitment to the war.
Mr McCain said he expected progress to be made in Afghanistan between now and July 2011. "But we must not tell the enemy that we will begin leaving when we have not finished the job," he said.
During a two-day visit, Mr McCain and Senator Lindsey Graham, a Republican from South Carolina who is on the Armed Services Committee, and Senator Joseph Lieberman, an Independent from Connecticut who is chairman of the Homeland Security and Governmental Affairs Committee, met General David Petraeus, the newly installed Nato commander, Afghan president Hamid Karzai and US ambassador to Afghanistan Karl Eikenberry.
Mr Lieberman said he understood that Mr Obama wanted to use the July 2011 timetable to send the message that the US would not be in Afghanistan forever, but thought the president was wrong to set it.
"We hear it everywhere we go here. They say they think we're leaving. We're not going to leave until we win," he said.
Mr McCain also said he expected General Petraeus to refine the rules of engagement on the battlefield.
"Probably there will be some tweaking," Mr McCain said. "We get that impression from him."
The rules were designed to limit civilian casualties and improve Afghan public support for coalition forces.
They do not prevent US troops from calling in air support, but the emphasis is on caution and some officers fear career damage if they mistakenly call for air or heavy weapons support and kill civilians in the process. Analysts say the rules have been interpreted and implemented unevenly across the country.