Back in January, Grace Pak had a simple question for the congressional commission planning the nation’s 250th birthday celebration.

Was anyone making a cake? The answer was no.

“How could we not have a cake for a birthday?” Ms. Pak wondered.

Thus began her odyssey to create America’s birthday cake, a sculpted work of bipartisan art the size of a Mini Cooper, adorned with flying chocolate Black Hawk helicopters, an eagle perched on a branch and Lady Liberty’s arm fashioned out of Rice Krispies Treats and green fondant. If she makes it over the last few hurdles, her masterpiece will be cut at a ceremony on Saturday in the Library of Congress.

Ms. Pak, 39, is a New York City-based cake artist whose work can be seen on the Food Network and at Plaza Hotel weddings. She knew that capturing a nation’s history and identity with flour and frosting would be no easy task, but she didn’t anticipate the political baggage.

“It’s the 250th, and I feel like so many people are not excited,” she said. “I understand where all of that is coming from. People aren’t excited because of what’s going on in the political landscape. But cake is the answer. Bring cake and everyone’s in.”

Like many family events, America’s birthday plans have gotten messy. The America250 commission, which Congress authorized in 2016 to plan the festivities, was elbowed aside last year by Freedom 250, a separate group created by President Trump, who wanted splashy events like an Ultimate Fighting Championship cage match on the White House lawn and a 40-minute July 4 fireworks show that he’ll kick off with what he has called a “Trump rally” on the National Mall. A cake was not among their plans.

The two entities barely communicate with each other. Though America250 signed on to her project, Ms. Pak had no budget to speak of. And to make the cake a significant part of the national celebration, she would have to get elected officials from both parties to buy in.

“I had my expectations set,” she said.

Cake has been part of America’s birthday celebrations from the start. A 1796 recipe for “Independence Cake” required 20 pounds of flour, a quart of yeast and copious amounts of wine and brandy. With its thick sugar frosting and flourishes of gold leaf, it proudly broadcast the prosperity of a young nation.

For the country’s 100th anniversary in 1876, the cake was lighter and more populist. Baking powder and baking soda were newly available to home cooks, who took to making spiced centennial cakes with molasses and a crumb topping instead of expensive sugar-based icing.

By the time the bicentennial rolled around in 1976, America had entered its cakemaxxing phase. In Philadelphia, the Sara Lee baking company erected five stories of scaffolding and covered it with thousands of sheets of frozen chocolate cake decorated with intricate historical scenes sculpted in royal icing. It was on display for three days, then 25 tons of cake were shoveled into wheelbarrows and hauled to dumpsters outside Memorial Hall.

In Baltimore, the city created a 35-ton birthday cake. The plan was to float it on a barge and sell slices for 50 cents to fund the civic celebration. The cake never made it off the dock. Temperatures soared and rats infested the cake. A sudden thunderstorm washed 3,000 pounds of icing into the harbor.

Ms. Pak is a relative newcomer to professional baking. She has a master’s degree in neuroscience from Columbia, and was working as a researcher when she lost three people close to her in one week. She reassessed her life and decided to attend culinary school.

“She wanted something day-to-day that was acutely fun,” said her husband, Dave Leffler, 39, who is a Marine.

Ms. Pak opened Duchess of Cameron, the cake studio she operates in New York and Aspen, Colo., in 2016. Her clients include Louis Vuitton, Tiffany & Company and the U.S. Marine Corps. That branch of the military is extremely serious about cake, celebrating its birthday each Nov. 10 with a ceremony in which the oldest Marine present is given a slice of cake, takes a bite and then hands it to the youngest, to symbolize the passing of knowledge and tradition.

The concept for the national birthday cake started to come together after Ms. Pak created the ceremonial cake for the Marines’ 250th birthday last November. She chose a theme — American Made — and created a budget that relied entirely on donations and volunteer hours from a Marine Corps culinary team.

“I wanted it to be a gift to the nation,” she said.

That the cake wasn’t going to cost them anything was music to the ears of America250 organizers. Congress had dedicated $150 million for the country’s birthday party, but the Trump-aligned Freedom 250 took most of it. America250 has so far received just $25 million.

“Her energy is incredible,” said Kristin Thompson, the deputy executive director of the America250 commission, who is especially excited that Ms. Pak has incorporated designs from winners of America’s Field Trip, a nationwide art and essay contest for students from third grade through high school.

The cake is four feet wide, and so long it would barely fit into a pickup truck with the tailgate down. She and her team made the components in a shared commercial kitchen in Washington, and will assemble it Saturday morning for transport. When all of the sculptural pieces are attached to metal rods sticking up from the base, it will be six feet tall — dimensions she had to choose so it will fit through the doors of the White House Visitor Center, where it’ll be displayed from July 7 to Aug. 2.

Anyone who has ever ordered a show cake for a big event knows that only a small section, called the cutting layer, is edible. Ms. Pak’s recipe for that layer started with deep research at Colonial Williamsburg and the Smithsonian Institution.

“At first I just couldn’t get my head around it,” said Paula Johnson, the curator of food history at the Smithsonian’s National Museum of American History. “But she is a serious person and has this larger belief in the power of food. Here we have our birthday, and who’s making the national birthday cake? Nobody. She leaped into the void.”

Ms. Pak procured freshly milled whole-wheat flour from George Washington’s grist mill at Mount Vernon, and butter from Vermont. She soaked currants in Kentucky bourbon, and flavored the cake with cardamom and thyme-infused honey. She layered in sautéed Granny Smith apple slices and covered the whole thing with Swiss butter cream.

Her showstopper is a ribbon that flows along the base of the cake, holding 56 squares of modeling chocolate, each the size of a dessert napkin, symbolizing a state, territory or the District of Columbia. There is a catfish for Mississippi, a sunflower for Kansas, a diner sign for New Jersey and the wide-brimmed straw hat called a pava from Puerto Rico.

Ms. Pak’s toughest challenge was getting members of Congress from both sides of the aisle involved. The idea was to have lawmakers give each square its final touches. She spent weeks making calls and running to Capitol Hill with special America250 aprons and edible paint. In the end, she corralled about 60 lawmakers, almost evenly divided between Democrats and Republicans.

She was pleasantly surprised to see how much fun they had. But she was shocked by the hatred people spewed in the comments under social media posts of the cake-decorating sessions. “I felt so bad after meeting these people,” she said of the officials. “They really seem to me to trying their best to do what’s right for the country.”

The battle was not over. The commission originally scheduled the cake ceremony for July 3 in a Capitol conference room because the building would be closed on the Fourth. Ms. Pak wasn’t having it. The birthday was July 4, not July 3. And her work would not be shoved into a conference room.

After a sternly worded email in which she argued that the setting devalued “the gravity of this historic milestone,” the ceremony was moved to the grand Madison Hall in the Library of Congress on July 4.

The invitation list included members of Congress, young scouts and donors, including some of the farmers who volunteered ingredients.

America250 sent the White House a memo weeks ago outlining all its plans, including the cake ceremony. But the cake’s existence came as news to some White House staff members when a reporter asked about it this week. As of Friday morning, no one from the administration was expected to show up for the party.