WASHINGTON, Mar 25 (Reuters) President George W Bush today tried to counter strong opposition among many Republicans to his guest-worker plan, arguing that allowing immigrants to take jobs Americans don't want adds to ''economic vitality.'' Bush weighed in on the immigration issue in his weekly radio address as the Senate was days away from taking up debate on the subject.

The president favors including a temporary work visa in broad legislation that would also bolster border enforcement.

But some Republicans view the guest-worker plan as a back-door amnesty for illegal immigrants and prefer an approach that focuses solely on toughening border security and cracking down on illegal immigration.

Bush, who this week urged all sides to tone down their rhetoric in the emotional debate, said securing borders was a top priority of immigration reform but invoked the country's history as ''a nation of immigrants'' to argue for a balanced approach.

''Comprehensive immigration reform requires a temporary worker program that will relieve pressure on our borders,'' Bush said.

''As we debate the immigration issue, we must remember there are hardworking individuals, doing jobs that Americans will not do, who are contributing to the economic vitality of our country,'' he added.

Senate Judiciary Committee Chairman Arlen Specter wants to finish work on legislation that includes a temporary worker program and would give undocumented aliens an opportunity to legalize their status.

The Pennsylvania Republican is working against a deadline set by Senate Majority Leader Bill Frist, a Tennessee Republican and potential 2008 presidential candidate. Frist plans to bring to the Senate floor his own border-enforcement immigration bill if the Judiciary Committee fails to pass legislation.

The panel is to meet on Monday in hopes of rushing legislation to the floor before Frist brings up his own bill.

POLITICAL PRESSURES Meanwhile, Bush is leaving on Wednesday for meetings in Cancun with Mexican President Vicente Fox, who has been disappointed by Bush's failure so far to achieve progress on the guest-worker program. Mexico took out full-page ads in US newspapers this week promoting the guest-worker idea.

Immigration is looming as a key issue in the November midterm elections in which Republicans are seeking to hold on to their majorities in both houses of Congress.

But the politics of border security have created competing pressures for Republicans.

Bush views the guest-worker program as way of courting Hispanic voters in key states like Arizona, New Mexico and Florida. But some conservative Republicans are focusing on enforcement as constituents vent frustration at what they see is a strain on schools, hospitals and other local resources from illegal immigration.

The president's weak approval ratings of around 36 per cent have led to an increased willingness among Republicans to rebel on issues like immigration.

Although the guest-worker plan would offer the estimated 12 million illegal immigrants a chance to register and work in the United States for up to six years, Bush rejected any description of it as an amnesty plan.

''I believe that granting amnesty would be unfair, because it would allow those who break the law to jump ahead of people who play by the rules and wait in the citizenship line,'' Bush said.

Reuters SHR BD2104