Democrats hope the bill, which will offer path to citizenship to 2m illegal immigrants, will win over Latinos before mid-term elections

The US Congress is poised to vote this week on a bill that offers more than 2 million young illegal immigrants a path to citizenship, a move that will re-open the toxic debate on the issue ahead of the November mid-term elections.

The vote could come as early as tomorrow.

The bill is being introduced by the Democratic Senate leader, Harry Reid, and is facing widespread opposition from Republicans.

The Democratic party sees it as a no-lose situation. If the Republicans vote against, the Democrats hope this will cement their position as the party of the Latinos.

A Republican senator, John Cornyn, accused the Democrats of being "cynical and transparently political" rather than interested in genuine immigration reform.

The Republicans and Democrats have over the last decade been battling to win over Latinos, the fastest-growing demographic group in the US, one that could provide a decisive electoral advantage this century. President George W Bush courted them but the Republican party over the past few years has adopted a harsher approach to illegal immigration in states such as Arizona and is in danger of alienating the Latino vote.

The bill would allow young illegal immigrants to become citizens if they have completed a university or college education or served two years in the military.

Barack Obama promised during this presidential run that he would introduce legislation to provide the estimated 12 to 20 million illegal immigrants, most of them Latinos, with a route to citizenship but has so far failed to deliver. This measure, even if the chances of passage appear at this stage to be slim, would go part way towards achieving that.

Obama, responding to anger from Latino leaders this month at his failure to deliver on his pledge, urged them to get out and vote in November, and portrayed the Democrats as their friends and the Republicans as their enemies. He told Latinos not to forget "who is standing with you, and who is standing against you".

The Pentagon, struggling to maintain levels of recruitment in the face of troop demands in Afghanistan, Iraq and elsewhere, is among the backers of the bill.

In a tactical device by the Democrats, the bill has been tacked on to legislation approving defence spending for next year, making it harder for the Republicans to obstruct it. If the defence spending approval is held up the Pentagon will have to seek emergency funding elsewhere.

The bill is labelled the Dream Act (Development, Relief and Education for Alien Minors) and insists that would-be students are of good moral character, came to the US before the age of 16, obtained a degree from an institute of higher education or "served in the uniformed services for at least two years and, if discharged, received an honourable discharge".

Supporters of the bill say that young people should not be penalised for the behaviour of their parents in entering the US illegally.

But opponents say it rewards lawbreakers. Dan Stein, president of the Federation for American Immigration Reform, which opposes the bill, said: "At a time when our country is at war and our troops are fighting and dying in Afghanistan, the use of a bill to authorise funding for our military to benefit illegal aliens exemplifies why the American public has grown contemptuous of the way today's Washington operates."

Reid, whose Democrats have a majority in the Senate, needs to secure 60 of the 100 votes to prevent Republicans blocking the bill. Some of his own Democratic senators are dithering over whether to back it, either because they do not want to alienate conservative voters in the election or because they do not think it goes far enough.

He has a personal interest in the issue, facing a tight race in Nevada that could depend on the Latino vote.

The proposed measure leaves the Republicans in a bind. If Republicans vote against the measure it will help the Democrats portray them as anti-immigration in the run-up to the elections in November in which a host of Senate and House seats and governorships will be at stake.

Most Republicans are planning to vote against.

The bill was first introduced in 2007 but failed at an early stage, partly because of a revolt of some Democrats. In a survey carried out by the Hill magazine, five of those Democrats said they were undecided about this week's vote.

One of them, Claire McCaskill, appeared to be shifting towards support, telling the Hill: "It depends on the language. I have some problems with the way the bill was drafted last time. I am certainly more comfortable with the notion that somebody who has been in the country for five years and who came here through the fault of their parents and not their fault ought to get a green card to serve in the military. I'm very sympathetic to that."

A Republican senator, Lindsey Graham, said: "This is rank politics. There's no way I'm going to vote for the Dream Act in isolation on the defence bill. And if they think I'm the problem, they're wrong. I will support good, comprehensive immigration reform, but not like this."