WASHINGTON — Donald Trump is the Republican Party's undisputed frontrunner, yet his White House aspirations may now depend on a messy fight for delegates he is only now scrambling to address.

Trump's campaign on Monday vowed to pursue legal action against the Republican National Committee to protect his recent victory in Louisiana, one of many states that feature complicated rules allowing campaigns to influence the presidential nominating process weeks or months after their votes have been counted.

A similar process plays out nationwide every four years. Yet Trump's outsider candidacy is so far driven largely by media coverage instead of the on-the-ground organization that his rival Ted Cruz boasts. Now, Trump must play catch-up — especially in the chase for delegates previously bound to former candidate Marco Rubio.

"A lot of Trump's support has been through earned media, so you haven't had the need to really focus on that aspect of it," said Jason Osborne, one of several former Ben Carson aides tapped in recent weeks to undertake Trump's delegate outreach. "But that doesn't mean you can't get up to speed pretty quickly on it."

Indeed, Trump's campaign on Tuesday will announce plans to open a Washington, D.C., office to run its delegate operation and congressional relations team, campaign senior adviser Barry Bennett said.

In addition to the new space, Bennett said, Trump has hired a veteran political operative to serve as the campaign's convention manager. Paul Manafort, a seasoned Washington hand with decades of convention experience, will oversee the campaign's "entire convention presence" including a potential contested convention, Bennett said.

The move marks a major escalation in Trump's willingness to play by party rules and build alliances in a political system he has so far shunned. It comes as Trump faces a Republican nomination battle that will almost certainly extend until the final day of primary voting on June 7 — or even to the party's July national convention in Cleveland — if he fails to secure the delegate majority needed to become the presumptive nominee.

In a campaign season so far defined by extraordinary insults and extreme rhetoric, the 2016 Republican presidential nomination fight could ultimately be decided by lawmakers, party activists, and lawyers.