BERLIN (Reuters) - Support for Germany's conservative bloc fell by three points in the days after its chancellor candidate said he would push through a migration crackdown with the backing of the far right, in a survey published four weeks before a federal election.

Trying to seize the initiative on asylum policy after an Afghan asylum seeker was arrested for a deadly knife attack targeting children last week, opposition leader Friedrich Merz vowed to close German land borders to irregular migration.

He also outlined plans to allow the federal police to request arrest warrants for people they apprehend who do not have the legal right to remain in Germany. Social Democrat (SPD) Chancellor Olaf Scholz and the Greens oppose the plans.

It is a gambit for Merz, the favourite to be Germany's next chancellor who has shifted his party's stance on migration to the right, as critics argue it breaks a taboo of working with the nativist Alternative for Germany (AfD), polling second.

Although Merz's conservatives are still poised to win most votes in the Feb. 23 election, in a Forsa poll on Tuesday their support had fallen three points to 28% from the middle of last week to Jan. 27.

Scholz's Social Democrats (SPD) gained two points to 17% as did the far-right AfD, at 21%.

Although some conservatives have signalled displeasure at Merz's plans, senior party member Thorsten Frei defended the move.

"We are at a point where we have to depart from tactical considerations. We have to do things that have to be done," he told reporters.

Scholz has already accused Merz of breaking the taboo on cooperating with the AfD, which is monitored by security services on suspicion of being right-wing extremist and has won the backing of U.S. billionaire Elon Musk.

Previously, the conservatives have only worked with the AfD at a regional and local level.

Stefan Marschall, political scientist at the University of Duesseldorf, said the strategy was for Merz to leave as little space as possible to the right for the AfD on the issues of migration and asylum.

"The risk was that the migration debate could turn into a debate about the firewall - and so it has," he said, adding that it gave the other parties ammunition to cast doubt on Merz's credibility and reliability.

As recently as November, Merz suggested to mainstream parties that no proposals would be put to the Bundestag lower house of parliament which needed the support of the AfD to be passed.

"Friedrich Merz is simply breaking his word," said senior Greens lawmaker Katharina Droege.

(Reporting by Andreas Rinke, Holger Hansen, Sarah Marsh, Writing by Madeline Chambers, editing by Ed Osmond)