CNN

(CNN) – A top campaign adviser to Senator John McCain recently told CNN they hope the presumptive GOP nominee's trip to the Mideast and Europe next week will illustrate his experience on the world stage, as Democrats continue to battle back home.

McCain will be joined by two of his most ardent supporters, Republican Sen. Lindsey Graham and Democrat-turned-Independent Joe Lieberman.

But McCain's visit abroad will be an official congressional trip - which means it will be paid for by taxpayers, not his campaign.

On Tuesday, McCain defended the situation, saying it is critical for him to travel to fulfill duties as ranking Republican on the Senate Armed Services Committee.

"I’ll be taking other members of the Armed Services Committee, and this will be one of many regular trips that I have taken over the last 20 years as a member of the Senate Armed Services Committee," said McCain.

"I think that it’s very important as the ranking member that I understand and keep up to date, particularly where young Americans may be in harm's way. And I’m proud to serve, and I regret that more of my colleagues don’t spend more time in foreign travel."

Senate Majority Leader Harry Reid declined to criticize the presumptive GOP nominee for traveling abroad at taxpayers' expense, but instead said he personally does not think it’s a good idea for senators to go to Iraq, as McCain and his entourage are planning to do.

“People have asked me, Why haven't I been back to Iraq?’” said the Nevada Democrat. “This is my personal feeling - only speaking for Harry Reid - when I went there to Iraq, I just felt the people who were there protecting me should have been protecting other people.”

He added: “I have the greatest respect for Sen. McCain, Sen. Graham and Sen. Lieberman” – who will be accompanying McCain to Iraq – “but I just - myself - am more focused on the eight soldiers killed yesterday” in attacks in that country.

When told about Reid's remarks, McCain was quick to respond, telling CNN though a spokeswoman that he thinks lawmakers responsible for funding the war also have a responsibility to see the situation on the ground, from the ground.

"I believe such firsthand knowledge is critically important to assess military and political progress in Iraq, and to make informed rather than political judgments about whether to continue our involvement there," said McCain.

"I learned enough on my first trip to Iraq to know that we were making serious mistakes, a conclusion borne out by my observations on later trips there,” said the Arizona senator. “I have also seen the astonishing progress we have made in Iraq since we changed commanders and our strategy."

–CNN's Dana Bash and Ted Barrett