SACRAMENTO, Calif. – The Bush administration has been unable to muster even half of the 2,500 National Guardsmen it planned to have on the Mexican border by the end of June.

As of yesterday, the next-to-last day of the month, fewer than 1,000 troops were in place, according to military officials in the four border states of Texas, California, New Mexico, and Arizona.

President Bush’s plan called for all 50 states to send troops. But only 10 states – including the four border states – have signed commitments.

Some state officials have argued that they cannot free up Guardsmen because of flooding in the East, wildfires in the West or the prospect of hurricanes in the South.

“It’s not a combat priority. It is a volunteer mission,” said Kristine Munn, spokeswoman for the Nation al Guard Bureau, an arm of the Pentagon, “so it’s a question of balancing the needs of the Border Patrol with the needs of 54 states and territories, and all those balls roll in different directions.”

Mr. Bush’s plan for stemming illegal immigration by using National Guardsmen in a support role called for 2,500 troops to be on the border by June 30, and 6,000 by the end of July.

But National Guard officials said yesterday that they probably won’t reach the 2,500 target until early to mid-July and won’t make the 6,000 deadline, either. Also, they said the number of troops will fluctuate from week to week over the course of the two-year mission.

“We now anticipate major waves in our deployment. There won’t always be 6,000. That will be the maximum,” Ms. Munn said.

Major problems began to appear last week when California, which has al ready committed to sending 1,000 troops, said it turned down an administration request for 1,500 more to cover expected shortfalls in the numbers sent by Arizona and New Mexico.