By Steve Holland

WASHINGTON (Reuters) - President Joe Biden on Thursday will use his last State of the Union address before the U.S. election to sharpen distinctions with Donald Trump on the economy and taxes on the rich, and to ease voter concerns about his age.

Biden's annual address, an event that stems from the U.S. Constitution's requirement that a president report to Congress "from time to time" on the state of the union, will be held at 9 p.m. (1400 GMT) before a joint session of the House of Representatives and the Senate, and a nationally televised audience.

It may be the Democratic president's biggest stage to reach the millions of voters weighing whether to vote for him, choose Republican challenger Trump or sit out the Nov. 5 election. Nikki Haley, Trump's last remaining rival for his party's presidential nomination, dropped out on Wednesday.

Opinion polls show Biden, 81, and Trump, 77, closely matched in the battleground states where voting preferences can swing either to Republicans or Democrats. Most U.S. voters, however, are not enthusiastic about either candidate or about a rematch between the two.

Biden will go into the well of the House chamber looking to convince voters that he is fighting to protect democracy from Trump - who continues to repeat false claims about his 2020 election loss and proposed jailing political enemies. Biden will also vow to protect abortion rights from Republicans, and lower costs for Americans facing high prices.

"You're going to hear the president address how democracy is under attack, how freedoms are certainly under attack," including women's reproductive rights and voting rights, White House press secretary Karine Jean-Pierre told MSNBC on Thursday.

Biden is set to renew his quest to make wealthy Americans and large corporations pay more in taxes, unveiling new proposals including higher minimum taxes for companies and Americans with wealth over $100 million.

Any such tax reform is unlikely unless Democrats win strong majorities in both houses of Congress in November, which is not forecast.

Biden also is set to promote his support for unions and lay out his legislative successes, for example on infrastructure and computer chips production, to show what is possible if he is given another four years in office.

He also will stress America's global leadership role and cite U.S. support for Ukraine and Israel as examples, an aide said. Biden is expected to use the speech to push, again, for a $95 billion aid package for weapons to Ukraine and aid to Israel that has been blocked by Republican House Speaker Mike Johnson.

Those priorities will also be reflected in the president's guests for the speech, including Swedish Prime Minister Ulf Kristersson, who is in Washington as Sweden formally joins NATO two years after Russia's invasion of Ukraine on Thursday and will be recognized in Biden's remarks.

Other White House guests include people impacted by in vitro fertilization or abortion restrictions, a veteran of the 1965 Bloody Sunday attack on Black marchers in Selma, Alabama, United Auto Workers President Shawn Fain and others.

Johnson, ahead of Biden's speech, on Wednesday said the president had damaged the U.S. economy and reputation: "Our stature has been diminished to a point that is difficult to calculate."

While U.S. Senator Katie Britt of Alabama will deliver Republican's formal response to the speech, Trump has said he plans to respond live online during Biden's remarks.