Donald Trump speaks during a campaign rally on April 6 in Bethpage, N.Y. (Julie Jacobson/AP)

GOP front-runner Donald Trump, facing new challenges in the wake of his second-place finish in the Wisconsin primary, announced Thursday that he is reorganizing his campaign, giving an expanded role to veteran strategist Paul J. Manafort.

With a growing possibility that Trump will not arrive at July's GOP convention having the 1,237 delegates needed to secure the nomination on the first ballot, his campaign will have to put more emphasis on the nuts-and-bolts, ground-level organizing at which his closest rival, Sen. Ted Cruz of Texas, has excelled.

That includes assuring that the delegates, many of whom will be picked in coming weeks at district and state conventions around the country, are people who will be loyal to Trump if the convention goes beyond a first ballot in selecting a nominee. Under GOP party rules, most delegates arrive at the convention bound by the results of their state primaries and caucuses only through the first round of voting; after that, they effectively become free agents.

Manafort, who joined the Trump operation on March 29 as its convention manager, will now have broader responsibilities, and will "oversee, manage and be responsible for all activities that pertain to Mr. Trump's delegate process and the Cleveland convention," the campaign said in its announcement.

Manafort, working from a new D.C. campaign office, will also be in charge of outreach efforts to members of Congress, the Republican National Committee and think tanks. This appears to be a move to thwart Cruz in his efforts to rally the GOP establishment behind him.

While the campaign insisted there had been no reduction in the role of embattled campaign manager Corey Lewandowski, it was nonetheless seen that way in Republican circles. Manafort's new portfolio is the operation that could be most crucial to Trump's success in the coming months.

Said one GOP strategist who has worked with Manafort, and who declined to be identified: "I can assure you that Lewandowski should be looking for new employment at this point. He's half wiped out already."

Manafort comes to the job with experience uniquely suited to the hand-to-hand combat for delegates that lies ahead. In 1976, which was the last time Republicans had a contested convention, he ran Gerald Ford's successful operation to secure enough delegates against insurgent Ronald Reagan to be nominated on the first ballot.

But Ford had the advantage of actually occupying the White House at that point. Trump has run an unconventional operation, with very little infrastructure and almost no support from the party hierarchy.

"Starting flat-footed is hard," the Republican strategist said. "They were completely unprepared for this phase of the campaign. I imagine Paul is scrambling around trying to figure out who their delegates are."

In a separate development, former New York mayor Rudy Giuliani said he’s voting for fellow New Yorker Donald Trump for president in that state's April 19 Republican primary.

“I support Trump. I’m gonna vote for Trump,” Giuliani told the New York Post Thursday.