Donald Trump pledged on Sunday to hire an extra 10,000 border patrol agents if he is reelected as president, as he intensifies his attacks on Kamala Harris on the issue of immigration.

The Republican presidential candidate said he would meet the goal by asking the U.S. Congress to fund a 10% pay rise for border patrol agents and a $10,000 retention and signing bonus, at a rally in the border state of Arizona, an election battleground.

Flanked on stage by leaders of the Border Patrol union, who have endorsed Trump, the former president said: "This will ensure that we can hire and keep the Border Patrol agents that we need."

Trump is locked in a close race with Democratic Vice President Harris ahead of the Nov. 5 election. Illegal immigration is a top voter concern, and Trump is seen by a majority of voters as the person best able to address it, opinion polls show.

Trump helped kill a bipartisan border security bill earlier this year that would have funded the hiring of 1,500 extra customs and border patrol agents, and an additional 1,600 asylum officers.

There are currently roughly 20,000 U.S. border patrol officers.

Harris, the Democratic presidential candidate, and President Joe Biden have both blasted Trump for his role in pressuring congressional Republicans to kill the bipartisan border security bill, accusing him of sabotaging it for political gain.

Some 7 million migrants have been arrested crossing the U.S.-Mexico border illegally during Biden's administration, according to government data, a record high number that has fueled criticism of Harris and Biden from Trump and fellow Republicans.

Harris has outlined her plans to fix "our broken immigration system" while accusing Trump of "fanning the flames of fear and division" over the impact of immigrants on American life.

Harris has also called for tighter asylum restrictions and vowed to make a "top priority" of stopping the deadly opioid fentanyl from entering the United States.

On Friday Trump called for the death penalty for "any migrant" who kills a U.S. citizen.

Trump has noticeably hardened his anti-immigration rhetoric in the final weeks of the campaign. Last month he called immigrants in the U.S. illegally who commit violent crimes "monsters," "stone-cold killers" and "vile animals."

Studies generally find there is no evidence immigrants commit crimes at a higher rate than native-born Americans and critics say Trump's rhetoric reinforces racist tropes.