Tory leader Michael Howard has stood by claims the UK could face race riots if people think immigration is unlimited.
Mr Howard's comments will raise the temperature on the subject
Mr Howard on Monday said he feared repeats of the 2001 riots in Burnley and Oldham unless people had confidence in controls on immigration.
On Tuesday, he denied waging a single issue campaign but said people must be "vigilant" for community relations.
Labour says it is improving immigration controls. The Lib Dems say the Tories are just targeting their core vote.
The comments come as three new opinion polls suggest the Conservatives are trailing Labour by between 5% and 9%.
Mr Howard denied reports in the Guardian and Times newspapers that some senior Tories raised fears at the weekend about his rhetoric on immigration.
The UK had benefited from immigration, he said, but he could not predict precisely what would happen if it was uncontrolled.
A report into the Bradford riots had warned about the pace of change, he said.
On Monday, Mr Howard was asked in ITV's Ask the Leaders debate if he feared more race riots in Britain "if there are more newcomers than is desirable".
The Conservative leader said: "Yes. I think people have to have confidence in the system. They have to understand there is a proper system of controls and that gives people reassurance.
"And I think that when people believe that there is no proper system, that immigration is out of control, I think that these anxieties make it more difficult to have good community relations."
Asked if he was warning of a repeat of violence, Mr Howard said he would not put it in those terms.
But he continued: "We have to be vigilant if we are to make sure we continue to have good community relations."
Asian gap year student Dean Velani, 18, accused Mr Howard of "pandering to xenophobia and hatred in our country".
But the Conservative leader said he "profoundly disagreed" and urged critics to say how they would deal with the problem.
"It does not take the debate much further to pin labels on me or abuse or insult me in the way in which you have just done," he said.
Gilbert Barthley said that, ever since he had come to Britain in 1954, he had heard campaigns based around immigration.
"When you talk in the street and you make these remarks you hear bits and that 'they are coming here for this, they are coming here for that'," he said.
Mr Howard said he regarded Mr Barthley as "a most valuable member of our national community".
Many other people from ethnic minorities supported his calls for controlled immigration, he added.
Mr Howard refused to comment on reports that media mogul Rupert Murdoch has given his support to Labour's policies on immigration.
Mr Murdoch is also reported to have said he was was not favour of an overall cap on immigration, a policy supported by the Tories.
On Monday, the Joint Council for the Welfare of Immigrants accused politicians of being short-sighted and potentially fuelling anti-immigrant sentiments.
"Immigration and asylum are issues that should be handled with care," he said.
The Lib Dems have attacked Labour and the Tories for engaging in a "Dutch auction" over immigration issues.
Leader Charles Kennedy said: "The Conservatives are trying to appeal to a core vote at the exclusion of sensible discussion."