Washington DC - The US Senate on Thursday rejected sweeping immigration reforms for the second time this year as Republicans again blocked what would have been the toughest border package in decades, dismissing it as a Democratic "gimmick."

Polling invariably shows immigration as a priority for voters ahead of November's presidential rematch between President Joe Biden and his Republican rival Donald Trump, with illegal crossings at historic highs.

The Border Act, hammered out by both sides over months of negotiations, initially had broad support but was killed by Republicans in February after an 11th-hour intervention from Trump, who is making the border a central campaign issue.

Democrats brought it back to the floor Thursday, where it fell at the first hurdle – a procedural vote to get the debate started – as Republicans again rejected the package.

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"People want us to get things done. People want us to come together," the Senate's Democratic leader Chuck Schumer said Thursday in a speech ahead of the vote.

"And when they hear that the only reason Republicans backed away from this bill is, not that it wasn't strong enough, but that Donald Trump said he wanted chaos at the border, they don't like that."

Republican leader Mitch McConnell had helped negotiate the package when it was attached to $61 billion in Ukraine aid and hailed it as a "huge success by any objective measure."

But he dismissed the plan to bring it back to the floor this week as "a gimmick" intended to boost Democrats in tight races and deflect from Biden's record on the border, which saw record illegal entries at the end of last year.

The vote came with Schumer desperate to cling to his 51-49 majority in the Senate but facing a tough map. The Republicans are defending just ten seats in November, while 23 Democratic seats are up for grabs.

Despite its failure, it served Schumer's goal of shielding his most vulnerable senators – all in districts where immigration is a top issue – from attacks that Democrats are soft on border security.

The bill – which calls for reform of the asylum system, extra staffing at the southern border, and emergency powers to shut it down when crossings meet a certain threshold – represented the strictest immigration crackdown in a generation.

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In the end, it could not even get a simple majority, let alone the 60 votes required to overcome blocking tactics in the chamber – as several Democrats also voted no.

The House passed a Secure the Border Act last year that Republicans say would end the crisis through measures including resumed border wall construction and the reinstatement of a Trump policy requiring asylum seekers to remain in Mexico.

The bill aims to purge the country of undocumented workers through mass surveillance while making it harder to claim asylum, slashing services to undocumented immigrants, and rolling back protections for migrant children.

Democrats argue that the sweeping piece of legislation amounted to one of the most draconian immigration bills Congress has ever seen and it has not been introduced in the Senate.

Schumer and Republican House Speaker Mike Johnson traded barbs over their competing bills as the Democratic senator decried "the definition of political theater" and the Republican congressman dismissed "a phony messaging exercise."