When President Donald Trump signed the National Defense Authorization Act in December, he did more than secure a funding increase for the American Defense Department, he made the establishment of a 355 vessel U.S. Navy a part of formal national policy. That provision, however, came with a significant caveat: no one seems to know how the U.S. can afford to get there.

Rep. Rob Wittman, one of the lawmakers that championed the provision, claims the 355 ship figure was established by conducting a macroanalysis of multiple fleet strength studies and determining what size Navy the United States really needs to stay the dominant power on the globe’s waterways.

However, despite the number of calculations that went into determining the figure, lawmakers devoted a good deal less effort to finding a way to pay for it — currently projecting that, even with the uptick in defense spending, it will take the Navy longer than they’re willing to formally project to reach that goal, though they’ve tossed out the round figure of 2050 for argument’s sake. Even that date, however, is widely considered to be tentative, as it would mean defense spending would need to remain consistently at its elevated levels through eight presidential elections and twice as many congressional ones.

Even with bipartisan support for increased defense spending today, it seems unlikely that Republicans and Democrats will still be seeing eye to eye on defense in every election for the next thirty years. Policies will change, as will executive directives and national security challenges. Domestic politics will evolve, and somewhere in the midst of all of that, the Navy hopes to continue to grow.