WASHINGTON — Veterans groups say it’s time to expand college aid for GIs, and Democrats want to use an election year to do it. Their biggest obstacle? The Pentagon.

The Defense Department is lobbying against legislation proposed by Sen. Jim Webb, D-Va., that would guarantee a full-ride scholarship for service members to any in-state public university. According to defense officials, the plan would hurt its ability to retain service members because the new GI education bill would require only three years before the full benefit kicks in. The Defense Department wants the commitment to be extended to at least six years.

“We have no issue with the fact that Sen. Webb wishes to provide a more generous education benefit to troops,” said Pentagon spokesman Geoff Morrell. “But we are certainly concerned that this would be eligible to them” so soon.

The Pentagon’s opposition to Webb’s bill underscores the difficulty the military has had in recruiting and retaining an all-volunteer force at a time when it is engaged in a war that is deeply unpopular with the American public.

Adding to the military’s dilemma is the larger number of soldiers and Marines needed to fight the wars in Iraq and Afghanistan. Last year, the Pentagon recommended that the Army be increased by about 65,000 soldiers to a total of 547,000, and the Marines be increased by 27,000 to 202,000.

The difficulty in finding young people also can be attributed in part to low unemployment numbers. According to the nonpartisan Congressional Research Service, unemployment dropped from 6 percent in 2003 to 4.6 percent in 2007.

In recent months, the military has had to take creative steps to reach its desired troop numbers. A year ago, when Army recruiters didn’t meet their goal, the service announced new $20,000 bonuses for recruits and up to $40,000 if an enlistee signed up for at least four years.

The Army also has granted special exceptions to recruits with prior criminal records, medical problems or low-aptitude scores that would have otherwise disqualified them from service. Senior military officials defended the change in policy as justified because they say current restrictions were so stringent that many members in Congress would have been denied entrance to the ranks because of indiscretions from their youth.

Retention rates have been less troublesome in the military, with the Army and Marine Corps exceeding their goals by large margins in 2006 and staying strong in 2007. Studies have found that combat deployments can prompt service members to re-enlist, usually because of a sense of accomplishment.

Still, the Defense Department is worried that its retention numbers could fall as service members are asked to return repeatedly to Iraq and Afghanistan and they are given too much of an incentive to leave. One particular problem facing the military is its ability to hang on to seasoned combat veterans, including those in the elite forces, who are being lured to higher-paying jobs in the private sector.

Webb, a Vietnam veteran and critic of the Iraq war, counters that his legislation would be more effective in attracting new recruits and would offset any drop in the military’s ranks.

“I can’t think of a better way to broaden (the) propensity to serve than to offer a truly meaningful educational benefit, rather than simply taking that smaller demographic” of those already enlisted “and pound on it” with repeated combat tours, he said.