[cnn-photo-caption image= http://i.cdn.turner.com/cnn/2008/images/05/22/art.mccainbill.ap.jpg caption="McCain campaigned in California Thursday as the Senate approved the GI Bill."](CNN) - Two presidential candidates were back on Capitol Hill Thursday, but the third was noticeably absent.
Presumptive Republican presidential candidate John McCain campaigned in California as his Senate colleagues voted to approve a measure that many Republicans and war veterans support – but he opposes.
The legislation, an updated version of the GI Bill, passed the Senate Thursday afternoon by a wide 75-22 vote margin and passed the House earlier this month by a similarly wide margin, proposes to essentially provide a full scholarship to in-state public universities for members of the military who have served for at least three years.
But McCain, as well as President Bush and much of the military brass, oppose the measure because they worry it will deplete retention rates among those currently serving in the military at a time when recruitment efforts are already struggling.
And in what is a sign of just how much of political issue this has become for the Arizona senator, his likely presidential rival Barack Obama took to the Senate floor earlier Thursday to directly chastise him for not supporting the measure.
The bill has already become a political football in the presidential race and poses a major dilemma for McCain: his experience as a post-Vietnam War military officer left him with firsthand experience of the effects of an understaffed military. But his current stand puts him in direct opposition of many veteran organizations whose support will be crucial to his White House hopes.
Democrats, including Obama, see the measure as an opportunity to undermine one of the Arizona senator's greatest strengths - his credibility on military matters and his broad support among military veterans.
In his speech on the Senate floor, Obama directly criticized McCain for opposing the measure.
"I can't believe why he believes it is too generous to our veterans," Obama said of McCain's opposition to the bill. "I could not disagree with him and the president more on this issue. There are many issues that lend themselves to partisan posturing, but giving our veterans the chance to go to college should not be one of them."
McCain responded forcefully, saying in a statement the Illinois senator has not taken the time to understand the issue. McCain also said it is "offensive" that Obama made such comments on the Senate floor.
"Perhaps, if Senator Obama would take the time and trouble to understand this issue he would learn to debate an honest disagreement respectfully," he said. "But, as he always does, he prefers impugning the motives of his opponent, and exploiting a thoughtful difference of opinion to advance his own ambitions. If that is how he would behave as President, the country would regret his election."
McCain's opposition to the measure is already a hot-button issue on the campaign trail.
Last month prominent Democratic veteran and former presidential candidate Gen. Wesley Clark - a supporter of Hillary Clinton - sent out an email asking "Why Won’t McCain Support Our Vets?"
The group VoteVets.org also launched a television ad earlier this week challenging McCain for opposing the bill.
The Arizona senator proposed his own version of the GI bill earlier this month that included more limited tuition breaks for military veterans. Specifically, McCain's bill proposed a sliding scale that offers increased benefits to veterans with longer lengths of service. McCain has said that lack of such a scale encourages people the leave the military sooner than they otherwise would.
That measure was killed by Senate Democrats last week.
"Most worrying to me, is that by hurting retention we will reduce the numbers of men and women who we train to become the backbone of all the services, the noncommissioned officer," McCain also said Thursday.
But McCain's arguments have not swayed the country's largest veteran organizations - including the Veterans of Foreign Wars and the American Legion, both of which strongly lobbied for the measure.
"This bill would encourage young men and women to join the military," National Commander Marty Conatser, the leader of American Legion, said earlier this month. "As far as retention goes, the Congressional Budget Office estimates that a simple $8,000 bonus to personnel at their first enlistment point would increase reenlistments by 2 percentage points."