HOUSTON (Reuters) -U.S. Vice President Kamala Harris vowed to empower labor unions and prevent school book bans while addressing a powerful teachers' union on Thursday, seeking to draw a sharp contrast with her Republican rival for the presidency Donald Trump.

In a 20-minute address in Houston to the American Federation of Teachers, Harris, 59, focused on economic policy and workers' rights, touting plans for affordable healthcare and child care and criticizing Republicans for blocking gun limits in the wake of school shootings.

"Today we face a choice between two very different visions of our nation, one focused on the future and the other focused on the past, and we are fighting for the future," she said. "Donald Trump and his extreme allies want to take our nation back to failed trickle-down economic policies, back to union-busting, back to tax breaks for billionaires."

Harris' visit to the AFT, which endorsed her earlier this week, continues a blitz of campaign appearances since President Joe Biden, 81, dropped his reelection bid on Sunday and urged Democrats to coalesce behind Harris.

"We want to ban assault weapons and they want to ban books," Harris said, a reference to the push by some Republicans to remove books that address gender and sexuality from some school libraries. The Second Amendment of the U.S. Constitution defends the right to bear arms.

Harris' move to the top of the ticket has shaken up a stagnant presidential race. A series of opinion polls conducted since Sunday, including one by Reuters/Ipsos, showed Harris and Trump beginning their head-to-head contest on roughly equal footing, setting the stage for a close-fought campaign over the next four-and-a-half months until the Nov. 5 election.

Trump, 78, on Wednesday night laid into Harris in his first rally since she replaced Biden atop the ticket.

He branded Harris a "radical left lunatic" after she had dominated the campaign the two previous days with withering attacks on him that pointedly raised his felony convictions, his liability for sexual abuse, and fraud judgments against his business, charitable foundation and private university.

Former President Barack Obama has been in regular contact with Harris and plans to soon endorse her as the Democratic presidential candidate, a source familiar with his plans said on Thursday.

Former U.S. House of Representatives Speaker Nancy Pelosi, who retains deep influence within the Democratic Party, endorsed Harris on Thursday alongside dozens of other female lawmakers.

After spending much of the campaign attacking Biden as old and feeble, Trump now faces a younger candidate in Harris the first Black woman and Asian American to serve as vice president.

The Harris campaign released its first video advertisement online on Thursday. Harris narrates the ad, framing the campaign as a battle to protect Americans' individual liberties to the sound of Beyonce's song "Freedom."

VEEPSTAKES

The next highly anticipated development will be Harris' choice of a vice-presidential candidate to counter Trump's selection of Ohio U.S. Senator JD Vance.

The list of contenders amounts to a who's-who of rising Democrats, including U.S. Senator Mark Kelly of Arizona, Governors Josh Shapiro of Pennsylvania, Roy Cooper of North Carolina and Andy Beshear of Kentucky, as well as Transportation Secretary Pete Buttigieg.

The Democratic National Committee's rules committee agreed on a plan on Wednesday to formally nominate Harris as soon as Aug. 1 - before the party's Aug. 19-22 convention in Chicago - with Harris picking a running mate by Aug. 7.

Harris' rise has largely pushed Trump out of the headlines, a week after the Republican National Convention and 12 days after he narrowly survived an assassination attempt that wounded his ear.

FBI Director Christopher Wray told a House panel on Wednesday that investigators are not certain whether Trump's injury was caused by a bullet or by shrapnel. Trump has said a bullet hit his ear.

A Trump campaign spokesperson, Jason Miller, called the idea that Trump was not hit by a bullet a "conspiracy," adding an expletive.