Barack Obama left Washington on a tour of swing states crucial to his re-election in November after delivering a populist state of the union speech that launched his 2012 campaign.

Obama called for a fairer, more equal America, the theme he is to pursue throughout the campaign, branding the Republicans as the party of the rich elite.

Republicans accused him of promoting "class warfare" and adopting "left-wing demagoguery".

Although Obama did not mention by name one of the Republicans chasing the party nomination to be his challenger, Mitt Romney was the president's target when Obama called for a minimal tax rate of 30% for millionaires.

"Washington should stop subsidising millionaires," the president said.

He added: "Now, you can call this class warfare all you want. But asking a billionaire to pay at least as much as his secretary in taxes? Most Americans would call that common sense."

Although Romney has an estimated wealth of $200m, he pays only 15% in taxes, a rate that is either lower than or the same as many working-class Americans.

Romney and the other frontrunner for the Republican nomination, Newt Gingrich, temporarily suspended attacking one another to denounce the president's state of the union address.

Gingrich, the former House speaker, addressing a mainly Latino audience in Miami on Wednesday, said the president's plan to raise the tax rate for millionaires to 30% was "typical left-wing demagoguery", would double capital gains tax and would halt job creation.

Gingrich had pressed Romney to release his tax details, which he did earlier this week. He said that unlike Obama, he did not want Romney to pay more than 15%, but wanted to reduce taxes so that everyone else only paid 15%.

"I am for smaller government. Obama is for bigger government."

Romney accused Obama of failing to deal with the major economic problems of unemployment and high debt, and railed against the president for his partisan approach.

"It's shameful for a president to use the state of the union to divide our nation," Romney said.

The central message that Obama is to take on the campaign trail was encapsulated in a single quote from the state of the union speech. "We can either settle for a country where a shrinking number of people do really well, while a growing number of Americans barely get by. Or we can restore an economy where everyone gets a fair shot, everyone does their fair share, and everyone plays by the same set of rules," he said.

The president also accused Congress of obstructionism in his attempts to reboot the economy and threatened to bypass them where he could. But many of the proposals he put forward, from comprehensive immigration reform to changes to tax, cannot be done without the agreement of Congress.

The list of proposals he set out was mainly a wish-list rather than likely to be implemented in the near future. It was more of a programme for a second term than policies likely to become reality in the coming months.

He concentrated mainly on domestic issues and only briefly touched on Iran and other foreign policy priorities, restating opposition to Tehran achieving a nuclear weapons capability. But he was populist enough to take credit for the killing of Osama bin Laden last year and said he treasured the flag that the Seal team had carried and signed for him.

The speech built on the theme of inequality that he first raised in an address in Kansas last month.

In the state of the union speech, he touched as many hot button issues as possible raised by voters, from the power of Wall Street to the pervasive influence of money in politics.

In what is Obama's final state of the union address before facing voters, he offered up policy proposals that included: support for clean energy; cutting red tape on construction jobs, the farming industry and elsewhere; help for college students; and mortgage cuts for homeowners.

Addressing the issue of Wall Street, he promised curbs, with the creation of a special unit to investigate the abusive mortgage lending that led to the collapse of the housing market. "I will not go back to the days when Wall Street was allowed to play by its own set of rules," he said.

One of the most emotional points came before he spoke, when entering the room he embraced Arizona congresswoman Gabrielle Giffords, who was shot by a gunman in January last year. Having missed the state of the union last year, she made a point of being present this year, her last public act before retiring later to concentrate on her recovery.

Obama received more than a score of standing ovations during the speech, though most Republicans did not join in, with senior figures such as John McCain, stern-faced, remaining in their seats.

On Iran, he claimed to have united much of the world behind the US approach, with crippling sanctions designed to dissuade Tehran from achieving a nuclear weapons capability.

He repeated the long-held US line that while he was pursuing diplomacy, a military strike remained on the table.

"America is determined to prevent Iran from getting a nuclear weapon, and I will take no options off the table to achieve that goal," he said.