WASHINGTON (REUTERS)

Vice President Kamala Harris is toughening her position on illegal immigration, taking on hardliner Donald Trump on his signature issue in a series of campaign events and digital ads in coming weeks, according to campaign staff.

The campaign plans to promote Harris' support for a bipartisan border security bill - defeated in the Senate in February after Trump came out against it - that would have increased funding for border agents and detention facilities, an official said.

Harris' more combative approach on immigration is expected to be on display as she campaigns around the country with her running mate Tim Walz, the governor of Minnesota, who was chosen in part for his appeal to voters in America's heartland.

A campaign official said Harris had a chance to reintroduce herself to voters after becoming the Democratic presidential candidate following US President Joe Biden's exit from the race last month. "It's all part of a larger effort by Harris to be direct and to go directly at Trump," said Matt Barreto, a pollster who has worked with the Harris and Biden campaigns. "Democrats always do well when they lean in on the immigration issue and don't run away from it."

The enforcement-first position is a departure from Biden's 2020 campaign, when he pledged a more humane approach to immigration than Trump. Biden gradually hardened his approach as illegal crossings increased.

Harris has made the border security bill a centrepiece of her platform, and a digital campaign ad has cast the election as a choice between "the one who will fix our broken immigration system. And the one who's trying to stop her." Jeffrey Jarman, a Wichita State University professor who focuses on political communication, said Harris' push-back was a way to avoid ceding the issue of border security to Republicans. "Failure to talk about the issue allows Republicans to completely dominate the discussion and frame her in the most unflattering way," he said. But he acknowledged that going on the offensive is unlikely to win over Republican voters and that Harris risked getting sidetracked on issues that are not as important to her supporters.

"Candidates who spend too much time talking about the issues of their opponent will always face a more difficult time winning the election," he said.

Numerous polls suggest voters have grown more concerned with high levels of illegal immigration under Biden. In a Reuters/Ipsos poll in May, some 45% of registered voters said immigration has made life harder for native-born Americans. Trump has promised to launch mass deportations if reelected.