Opposition CDU leader Merz and his Bavarian CSU allies have vowed a crackdown on irregular migrants but provoked strong protests in parliament and on the streets after accepting support from MPs of the far-right Alternative for Germany (AfD).
Their joint votes in parliament twice last week breached a long-standing taboo in post-war Germany and shattered what has been labelled a "firewall" of non-cooperation between mainstream and extremist parties.
Mass rallies were held in Berlin and other cities on Sunday with tens of thousands of protesters waving signs such as "Shame on you, CDU" and "We are the firewall".
The controversy has redefined the campaign ahead of the February 23 election and allowed centre-left Chancellor Olaf Scholz and the Greens to attack Merz, who also saw disunity within his own ranks after influential ex-chancellor Angela Merkel strongly criticised him.
Having kept a relatively stable lead in opinion polls for weeks, the CDU and their top candidate for chancellor, Merz, had looked to be cruising towards victory.
But the outrage over their taboo-breaking decision to vote together with the AfD in the Bundestag has injected uncertainty into the campaign.
Merz, who has set about moving the CDU to the right away from the policies pursued under more centrist Merkel, has shown no sign of recanting his approach.
"I am giving voters in Germany the guarantee that there will be a real turnaround in economic policy and asylum policy," Merz told the Bild newspaper on Sunday.
Far right in government?
At Monday's CDU party congress in Berlin - where protesters again rallied outside the venue, holding signs calling the party's work with the AfD a "disgrace" - Merz was expected to double down on his stance.
CDU delegates were expected to support a 15-point plan to implement "immediately" if the party wins the elections and forms a government.
It contains many of the measures in one motion passed with the AfD, including plans to turn back asylum seekers at the German border and the unlimited detention of "dangerous" migrants who are set for deportation.
Merz and his party also want to emphasise action to revive Germany's stagnating economy, including tax cuts and slashing red tape.
The CDU/CSU alliance has been polling at around 30 percent in the run up to the election, but has seen the far right creeping up behind on around 21 percent.
Scholz's Social Democrats, who have led condemnation of Merz's parliamentary manoeuvres, sit third in the polls on around 16 percent.
Scholz has aired a suspicion that the conservatives and the AfD could enter into government together after the elections, a possibility strongly denied by the CDU leadership.
The AfD has meanwhile argued that voters should choose "the original", not the CDU, if they really want to see action on immigration.
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