Mayor Bloomberg is taking on President Obama for not getting aboard the crusade against illegal guns.

In a harsh March 3 letter, the mayor urged the president to move the gun issue to the top of his agenda or risk responsibility for lives lost to ongoing gun violence.

“Further delay will almost certainly result in the needless loss of innocent lives, including many children,” the mayor wrote in a letter co-signed by Boston Mayor Thomas Menino. “Mr. President, the time has come for action.”

A letter the two sent last August to top administration officials, pleading for enforcement of existing gun laws, resulted in a meeting between the mayors’ group and the Justice Department.

But there hasn’t been enough movement to suit the mayor.