President Obama is planning to invest heavily in modernizing the nation's nuclear defense as weapons built up during the Cold War reach the end of their service life, but critics are questioning the timeline and budget.

The administration's plan to rebuild America's aging nuclear assets would cost about $348 billion from 2015-24, according to an estimate from the Congressional Budget Office. Independent estimates have put the cost at about $1 trillion over 30 years.

Analysts say the price tag and timeline are just not realistic in today's budget environment, especially when most major projects face delays and cost overruns. The bulk of spending on nuclear programs, including replacements for Ohio-class ballistic missile submarines and the Northrop Grumman long range strike-bomber, will come in the mid-2020s. Yet that will occur as the services pay for other modernization projects, like Lockheed Martin's F-35 joint strike fighter and Boeing's new KC-46 Air Force tanker.

All these bills coming through at the same time make a large-scale modernization of the nuclear arsenal "un-executable," said Kingston Reif, the director for disarmament and threat reduction policy at the Arms Control Association.

"The cost to do this is just enormous, and many of the costs are going to be overlapping and peaking at roughly the same time," Reif said. "It's just a perfect storm, a budgetary train wreck."

The administration is planning to modernize its stock of aging nuclear weapons, as well as the systems that deliver them. The Pentagon awarded an $80 billion contract last year to Northrop Grumman to build the Air Force's next stealth bomber.

The Ohio-class submarine replacement program, a joint effort between General Dynamics Electric Boat and Newport News Shipbuilding, is expected to cost about $100 billion for 12 planned hulls. The current Ohio-class ballistic missile subs will begin retiring in 2027.

In addition to the high costs, Jeffrey Lewis, director of the East Asia nonproliferation program at the James Martin Center for Nonproliferation, said the tight time schedule means the planned number of ships will likely be cut due to delays.

"The money is going to be a problem, but on top of that, everything is on such a tight schedule that it would still be really hard to do even if there was enough money," he said. "No slack in the modernization schedule is a recipe for disaster."

Obama's planned investment in nukes is at odds with past rhetoric that sought to eliminate all nuclear weapons.

"I state clearly and with conviction America's commitment to seek the peace and security of a world without nuclear weapons," the president said during a 2009 speech in Prague.

The president's vision of a world free from nukes was cited as one reason for him receiving a Nobel Peace Prize in 2009.

More recently, the nuclear deal with Iran sought to prevent the country from developing a nuclear weapon and prevent the number of nukes around the world from growing.

But some analysts say Obama's actions in the White House have not matched the rhetoric.

"It's a nice speech to give and I think he does care about things like nuclear security, but when the administration came in and did a nuclear posture review, it didn't represent a dramatic change," Lewis said.

Because most of the modernization programs are still in early research and development phases, there's still an opportunity for the next president to reshape the modernization plan if he or she has a different idea.

"It's really the next president that's going to be staring at the enormous challenge of recapitalizing our force given where our current plans are going," Reif said.

Most candidates to be the next commander in chief don't have a very detailed nuclear strategy. Reif said Sen. Marco Rubio, R-Fla., has the most detailed platform, which calls for continued procurement of the Ohio-class replacement and long range strike-bomber as well as modernization of the nuclear arsenal.

On the other side, Sen. Bernie Sanders, I-Vt., has been vocal that the U.S. has far more nuclear capability than it needs and that large budget savings can be found by cutting those programs.

Others haven't addressed the issue, choosing instead to focus on Russia and the threat from the Islamic State. When asked which piece of the nuclear triad he would prioritize as commander in chief during the last Republican debate, front-runner Donald Trump stumbled, leading some to believe he didn't actually know what the nuclear triad was.

"I think, for me, nuclear is just the power, the devastation is very important to me," he said.