WASHINGTON — As a candidate, President Trump billed himself as a new breed of think-big Republican, pitching a $1 trillion campaign pledge to reconstruct the nation’s roadways, waterworks and bridges — along with a promise to revive the lost art of the bipartisan deal.

In the White House, Mr. Trump has continued to dangle the possibility of “a great national infrastructure program” that would create “millions” of new jobs as part of a public-private partnership to rival the public works achievements of Franklin Delano Roosevelt and Dwight D. Eisenhower. He chastises anyone who forgets to include it near the top of his to-do list, telling one recent visitor to the Oval Office, “Don’t forget about infrastructure!”

But an ambitious public works plan, arguably his best chance of rising above the partisan rancor of his first six months in office, is fast becoming an afterthought — at precisely the moment Mr. Trump needs a big, unifying issue to rewrite the narrative of his chaotic administration.

Infrastructure remains stuck near the rear of the legislative line, according to two dozen administration officials, legislators and labor leaders involved in coming up with a concrete proposal. It awaits the resolution of tough negotiations over the budget, the debt ceiling, a tax overhaul, a new push to toughen immigration laws — and the enervating slog to enact a replacement for the Affordable Care Act.