The event, which begins Wednesday and runs through Sunday, will bring together the nation's best swimmers as they vie for spots in the 2015 FINA World Championships in Kazan, Russia. Analysts will have their eyes on Phelps, not only to see how he fares against world-class competition but to glean a hint of which events he hopes to swim at the 2016 Olympics in Rio de Janeiro.
This week, he's entered in the 100 butterfly, 100 freestyle, 100 backstroke and 200 individual medley.
"I think he'll be swimming most of the events you'll probably see him swim going forward," said Phelps' longtime coach, Bob Bowman. "It's a good test for him. He'll try to step it up another level."
Phelps acknowledged in pre-meet remarks Tuesday that his planned four-race schedule and the elite competition will make nationals the stiffest test of his comeback.
The Rodgers Forge native said he still harbors significant goals, though he was coy about them as usual.
"There are always things that I still want to do and still want to achieve, and that's part of the reason why I'm still here," Phelps said. "You're not going to get what it is."
Phelps hasn't publicly committed to aiming for a fifth Olympics. But the swimming world assumes that's his intention, given his decision to throw himself back into the daily drudgery of training after a 20-month retirement.
"This is an important summer for him because it does set up the next couple of years," said NBC swimming analyst and former Olympic gold medalist Rowdy Gaines.
The early stages of his comeback have revealed a Phelps who can still swim some of the fastest times in the world but who's less inclined to the rigorous event schedules that defined him in his prime.
He's training less than half as much, Bowman said. And Phelps has said he's enjoying himself more because he's setting his own schedule. He has spoken repeatedly of the fun he's having, a far cry from the run-up to the 2012 Olympics, when training became a chore.
Phelps grabbed the attention of swimming insiders with his most recent win in the 100-meter butterfly at the recent Bulldog Slam in Georgia. His time of 51.67 seconds might not seem like much, given that Phelps holds the world record at 49.82 seconds. But the time would've been good for fourth at the 2012 Olympics, and Phelps produced it at a relatively casual meet less than six moths after he returned to serious training.
"That's pretty jaw dropping," said Mel Stewart, a former Olympic gold medalist who analyzes the sport through his website, SwimSwam.com. "It's the kind of thing where, if swimming people are sitting around having a few beers, they'll look at that and say, 'Whoa!'"
The time prompted Stewart to wonder if Phelps, no longer burdened by the strength-sapping 400-meter IM, might be headed for his fastest swims ever in the shorter races. It's a potentially unsettling thought for rivals such as South African butterfly master Chad le Clos.
"We never saw just how fast he could be, the pure speed he could achieve," Stewart said of Phelps. "But now we're seeing it. He might still reach new heights."
The possibility is enhanced by the greater competition Phelps faces in daily training at the Meadowbrook Aquatic Center in Mount Washington. Bowman has assembled something of a dream team, with Olympic gold medalists Yannick Agnel, Allison Schmitt and Conor Dwyer joining rising stars such as Chase Kalisz.
"They all know they're going to be tested in practice every day," Bowman said. "They can't just slough it off."
By his own reckoning, Phelps has delivered a mixed bag of performances in his return to competition.
He seemed pleased when he finished second in 52.13 seconds in the 100-meter butterfly final at his first event, April's Mesa Grand Prix. Despite several slip-ups in technique, Phelps held his own with longtime rival Ryan Lochte and other top competitors.
Within a few strokes, Stewart said, it was obvious Phelps' comeback wouldn't be a disappointment akin to the aborted return of his onetime rival, Australian gold medalist Ian Thorpe.
But Phelps was less enamored with his performances at the Charlotte Grand Prix in May and the Santa Clara Grand Prix in June. His time in the 100-meter butterfly, the one race he's entered in every meet this year, stagnated. And Phelps' tone verged on annoyed as he discussed the swims.
Gaines said no matter how much Phelps talks about being more relaxed, signs of his old competitive fire have emerged. Before the Mesa meet, for example, Gaines said it didn't really matter how Phelps performed because his legacy was safe regardless. On the deck at the event, Phelps — always looking for slights to use as competitive fuel — walked up to the analyst and said, "So, you think I'm going to swim crappy, huh?"
"By the time we went to the meet in Georgia, I think he was definitely a little more serious, Bowman said.
Phelps broke through at the Bulldog Slam, where he lowered his butterfly time to second best in the world this year. His first 50 meters, usually not his strength, were particularly fast. He won the event in 51.21 seconds at the 2012 Olympics in London.
"I am very pleased with being able to go 51," Phelps said after the race. "But in the grand scheme of things, I think it's just a small steppingstone to go where we hope to be."
Phelps also won the 100-meter backstroke at the Bulldog Slam with one of the 15 best times in the world this year and finished second to Agnel in the 100-meter freestyle, a swim he described as "blah."
'Can he still win?'
Phelps' leaner racing program could work to his advantage.
Gaines believes Phelps, who will be 31 by the Rio Olympics, could become better than ever in the short races as he focuses on details of sprinting technique rather than spreading himself thin.
"This has got to be tough to watch for his competitors," Gaines said. "They're thinking, 'God, I've been in the pool for the last two years, and he's swimming this fast after all that time away.'"
Regardless of his performance in the pool, Phelps remains unique among American swimmers for the attention he attracts outside the Olympics.
As soon as he announced his comeback in April, ticket sales spiked at his potential events. The New York Times and ESPN covered his first swim in Arizona. In the ensuing months, he appeared in a Subway commercial with Brazilian soccer legend Pele that ran in heavy rotation during the World Cup and posed as a cover boy for ESPN The Magazine's "body issue."
Tuesday, he announced a new swimwear partnership with Aqua Sphere.
From adolescent fans to grizzled rivals, everyone has raved about having Phelps back in the sport. Lochte even said it broke his heart not to compete against his old pal during Phelps' 20-month retirement.
But inevitably, the questions about his comeback have shifted from: "Will he?" to "Why did he?" to "Can he still win?"
Phelps knows better than anyone that it all comes back to winning in the pool. That's why he's spent several weeks in May at Bowman's high-altitude boot camp in Colorado, training he dreaded even when he was in peak condition.
He could have stuck to golf and exotic travels as he did in the wake of the 2012 Olympics. Instead, he's back to the grind.
Phelps at U.S. Nationals
Michael Phelps is entered in four events at the U.S. nationals this week in Irvine, Calif. Here's a look at his schedule:
Wednesday: 100-meter freestyle
Friday: 100-meter butterfly
Saturday: 100-meter backstroke
Sunday: 200-meter individual medley
Prelims start daily at noon (Eastern), finals start daily at 9 p.m.