Former defense secretary Leon E. Panetta speaks during a discussion on countering violent extremism on Oct. 23 in Washington. (Drew Angerer/Getty Images)

Three former secretaries of defense are warning lawmakers not to enact proposed Republican tax restructuring plans, arguing they will jeopardize future military spending.

Former defense secretaries Leon E. Panetta, Chuck Hagel and Ash Carter told senior congressional leaders in a letter Wednesday that because the tax plan is expected to increase the debt, passing it will probably mean future cuts to Pentagon budgets “for training, maintenance, force structure, flight missions, procurement and other key programs.”

“The result is the growing danger of a ‘hollowed out’ military force that lacks the ability to sustain the intensive deployment requirements of our global defense mission,” the secretaries wrote. They cited two recent accidents involving U.S. Navy destroyers that led to the deaths of 17 sailors as evidence that cuts in military spending can lead to a “lack of adequate training.”

The trio blasts “a broken budget process in Congress” for leaving the Pentagon with “a lack of certainty as to what budget resources will be provided for defense and other national security requirements in the next year.”

Several lawmakers, including Congress’s most influential hawks, routinely point to the budget process as a threat to national security. They say the budget caps Congress imposed on itself in 2011 are senselessly restricting the Defense Department’s ability to replace broken and outdated equipment, offer adequate training to service members in high-risk situations, and are putting the United States at a competitive disadvantage against its international rivals.

Democrats and Republicans banded together this year to endorse a nearly $700 billion spending plan for military programs and operations to try to make up for some of those shortfalls. But it is all but universally acknowledged that appropriators will not find the funds to cover the programs they authorized.

Democrats have argued that contemplating the GOP’s tax-cut plans in the current environment is simply adding insult to injury for the military.

“We are having this debate now and talking about how underfunded the military is … but the rest of this same week, Congress is trying to make sure our government takes in trillions of dollars less money through tax cuts. That is wildly inconsistent,” Rep. Adam Smith (D-Wash.), the House Armed Services Committee’s top Democrat, said this week. “If we believe we have these needs, we ought to be able to pay for them.”

The former Pentagon chiefs struck a similar tone in their letter Wednesday.

“Tax relief without fiscal discipline will inevitably add to the national debt,” they wrote. “That increase in the debt will, in the absence of a comprehensive budget that addresses both entitlements and revenues, force even deeper reductions in our national security capabilities.”

Former defense secretary Chuck Hagel takes a snapshot with veterans before ceremonies to commemorate the 35th anniversary of the Vietnam Memorial on Veterans Day on Nov. 11. (Bill O’Leary/The Washington Post)

In his own statement, Panetta said, “The proposed tax plan will cause the deficit to skyrocket and will deprive us of the resources we need for defense. This plan is both unwise and dangerous.”

Panetta, Hagel and Carter all served as Pentagon chief under former president Barack Obama, but each maintains close ties to both Democratic and GOP lawmakers. Panetta is a former California Democratic congressman and White House budget director, while Hagel is a former Republican senator from Nebraska. Carter held assistant secretary and deputy roles at the Pentagon until Obama and during former president Bill Clinton’s administration.

Their letter is addressed to top House and Senate leaders plus the leaders of the House and Senate Armed Services committees. Democrats, led by Senate Minority Leader Charles E. Schumer (D-N.Y.) and Sen. Jack Reed (D-R.I.), the top Democrat on the Senate Armed Services Committee, plan to tout the concerns of the former defense secretaries at an event on Thursday at the U.S. Capitol.