Joseph Biden, the top Democrat of the Senate Foreign Relations Committee, made the prediction after new data released by the Pentagon showed that the US army is failing to meet its recruitment targets for four straight months.
"We're going to have to face that question," Biden said on NBC's Meet the Press television show when asked if it was realistic to expect restoration of the draft.
"The truth of the matter is, it is going to become a subject if in fact there's a 40% shortfall in recruitment. It's just a reality," he said.
The comment came after the Department of Defence announced on Friday that the army had missed its recruiting goal for May by 1661 recruits, or 25%. Similar losses have been reported by army officials every month since February.
But experts said even that figure was misleading because the army has quietly lowered its May recruitment target from 8050 to 6700 people.
That has prompted charges that the real shortfall was closer to 40%, which in turn has led to questions about the future viability of the army as a force if it continues to be plagued by lack of new recruits.
Since October, the army has recruited more than 8000 fewer people than it had hoped to, which amounts to a loss of about a modern brigade.
The war in Iraq has severely
stretched the US military
The army, navy and marine corps reserves also fell short of their monthly goals by 18%, 6%and 12% respectively, according to the figures.
Recruitment at the Army National Guard was down 29% while the Air National Guard fell short by 22%.
The United States abandoned the military draft in 1973, after mass protest during the Vietnam War, and switched to an all-volunteer force.
Mandatory registration for the draft was suspended in 1975, but resumed in 1980 after the Soviet invasion of Afghanistan. About 13.5 million men are currently registered with the US government as potential draftees.
"If you think you have trouble getting recruits today, you're going to have far more trouble six months from now"
Patrick Leahy, US senator
During the 2004 election campaign, Democratic presidential nominee John Kerry repeatedly accused President George Bush of planning to re-instate "a back-door draft", charges the president vehemently denied.
But while admitting that restoring the draft would be politically "very difficult," Senator Patrick Leahy, the ranking Democrat of the Senate Judiciary Committee, said something would have to be done because the situation with recruitment was not likely to improve.
Severe problems ahead
"If you think you have trouble getting recruits today, you're going to have far more trouble six months from now," Leahy predicted on CBS's Face the Nation programme. "It is not going to get better. That's going to get worse."
Republican Representative Curt Weldon called the recruitment shortfalls "troublesome" and "unacceptable."
But he urged the military "to find ways to fix the current system" and to attract more recruits with the help of new incentives.
Nearly 1900 US troops have been killed in Iraq, Afghanistan and elsewhere since the beginning of the war on terror in the wake of the 11 September 2001 attacks.