Romney promises not to revoke temporary visas for young migrants, as he looks to erode president's lead among Latinos

Mitt Romney has softened his position on immigration, promising for the first time to honour changes introduced in the summmer by Barack Obama that offer temporary visas to undocumented young people brought into the US by their parents.

In an interview with the Denver Post before the first presidential debate in Colorado on Wednesday, he pledged that he would not revoke Obama's executive order and that undocumented young people who were brought to the US before their 16th birthday will not be deported.

Romney has avoided answering the question of what he would do with Obama's order if he won the election, saying only that the issue had to be addressed by long-term reforms. His shift, on the eve of the debate and five weeks before the White House election, is aimed at cutting into Obama's commanding lead among Latino voters, who, though a minority, could tip the balance in some swing states.

Romney told the paper: "The people who have received the special visa that the president has put in place, which is a two-year visa, should expect that the visa would continue to be valid."

He added: "I'm not going to take something that they've purchased. Before those visas have expired we will have the full immigration reform plan that I've proposed."

His change came the day before the first presidential debate, being held in Denver, and five weeks before the White House election.

Romney took a hard line on illegal immigration during the Republican primaries and caucuses, saying he would pursue tough policies that would lead to "self-deportation". But he now faces electoral arithmetic that shows Latinos backing Obama by at least two to one – and that could make a difference in swing states such as Colorado, Nevada, Virginia and Florida.

When Obama introduced the change in June with an order that bypassed Congress, Romney criticised the president for failing to go through the legislature. Faced with questions at the time about what he would do about the visas if he won the election, Romney opted for ambiguity, saying the order would be "superseded" by what he said would a comprehensive immigration reform package.

There are an estimated 11 million or more undocumented migrants in the US, making immigration an emotive issue. Some diehard Republicans favour deporting those immigrants without papers, but Democrats, and some moderate Republicans, argue this is impractical and needs a more humane approach.

Obama won the support of Latinos in the 2008 election, but disillusionment crept in with his failure to deliver on immigration reform combined with a continuing high number of deportations.

In an interview last month with Spanish-language network Univision, he described the lack of immigration reform as the biggest failure of his first term. His order lifting the threat to undocumented children rallied Latinos behind him again.

Romney and some of the other Republican presidential candidates alienated many Latinos during the primaries and caucuses with their anti-immigrant rhetoric. Only Newt Gingrich and Rick Perry took a different line.

Since becoming the Republican presidential nominee, Romney has tentatively tried to woo Latinos, with a series of Spanish political broadcasts and appearances on Spanish-language TV stations.

Romney had been working on an immigration reform plan with the Florida senator Marco Rubio, but abandoned it when Obama announced his order.

In the Post interview, Romney said he would introduce immigration reform in his first year in office but did not go into specifics.

"I will propose a piece of legislation which will reform our immigration system to improve legal immigration so people don't have to hire lawyers to figure out how to get here legally," Romney said. "The president promised in his first year – his highest priority– that he would reform immigration, and he didn't. And I will."

As well as voting for the president in November, Colorado will also vote on whether to legalise marijuana, allowing people to possess small amounts for recreational purposes, the first state in the US to allow this. At present, it is available over the counter in the state for medicinal uses.

The change is backed by some Republicans as well as Democrats.

A recent poll showed 51% in favour of the change in law but Romney expressed his opposition.

"I oppose marijuana being used for recreational purposes and I believe the federal law should prohibit the recreational use of marijuana," he said.