President Barack Obama hailed the selfless dedication of the US military's "9/11 generation," on a scorchingly hot July 4 holiday on Wednesday as families gathered to celebrate the nation's Independence.

In a speech steeped in patriotic fervor barely four months before a general election, Obama hosted a barbecue and fireworks at the White House, seeking to lift the mood after storms forced many US states to cancel their celebrations.

More than a million customers remain without power five days after hurricane force winds tore across the country, wreaking havoc from Illinois to Ohio to Kentucky, and causing destruction in and around the capital Washington.

The consequent lack of air conditioning made July 4 a miserable day off for many but Obama focused his attention on the military, while praising Americans for organizing July 4 family gatherings.

"Happy Fourth of July everybody," the president, alongside First Lady Michelle Obama, told his guests on the White House lawn, thanking all three armed services plus the US Coast Guard for their dedication to duty.

"Today, all across America, at schools, and beaches, and in town squares, Americans are celebrating the freedoms that all of you and your families defend. Like many of them, we're grilling in the backyard," Obama said.

"All the men and women who stand with us here this afternoon are an example of this generation of heroes -- this 9/11 generation that has earned its place in history alongside the greatest generations."

While the capital's massive fireworks show was due to start at 9:10 pm (0110 GMT), other displays were canceled due to the chaos caused by last week's fatal storm -- 20 deaths have been attributed to the weather.

Falling trees snapped power lines and companies struggled to get power back to many families, sweating again in sweltering conditions on Wednesday as the mercury soared back above 90 degrees Fahrenheit (32 degrees Celsius).

West Virginia, one of nine states heavily impacted, had the highest number of customers without power at roughly 317,000, according to a US Department of Energy statement on Wednesday morning.

Traditional barbeque events had to be re-arranged as partygoers looked for respite in cooling shelters.

The National Weather Service, meanwhile, warned of large hail and damaging winds in the Great Lakes region, especially in northern Minnesota, potentially throwing a wrench into festivities there.

In Chicago, the sizzling heat could make this year's holiday the hottest in a century.

Elsewhere, in arid areas such as the western state of Colorado where wildfires have been raging, firework celebrations, an annual draw for young and old alike, were scrapped to avoid sparking more blazes that already left hundreds homeless.

The US election heat was also intensifying as Obama and Republican rival Mitt Romney earlier sought to capitalize on the patriotic undertones of the day, which marks the 236th year of independence from Britain.

Obama held a naturalization ceremony for active duty members in the White House mid-morning.

"No other nation constantly renews itself, refreshes itself with the hopes, and the drive, and the optimism, and the dynamism of each new generation of immigrants," Obama told the group.

His speech to people who had immigrated from as far afield as Guatemala, Nigeria and Ukraine was a clear nod to a recent shift in his administration's immigration policy that could help the Democrat secure a second term.

Romney, taking a week off to spend time with his family at an upscale lakeside property in Wolfeboro, New Hampshire, was due to participate in a parade in the New England town as cameras rolled.

"As we gather to enjoy food and fireworks, let us also take a moment to pay tribute to the patriots who signed the Declaration of Independence -- and the brave men and women who have fought to protect our freedoms through every generation that has followed," he said in a statement.

"With so many around the world still consigned to tyranny, the Fourth of July is a time to appreciate the blessing of liberty and be thankful that we are Americans."

Despite the dire scenarios faced by many, large scale celebrations were still going on in New York and Boston where singer Jennifer Hudson was set to perform.

Headlines of quirky Fourth of July traditions also were not to be missed.

An annual hotdog-eating competition in New York, for one, saw the male and female winners down 68 and 45 frankfurters respectively.