(CNN) German Chancellor Angela Merkel has ruled out setting an upper limit on refugees coming to Germany, defying her long-term coalition partner on an issue that threatens to open up a rift between the country's ruling political parties just two months before federal elections.

"On the issue of an upper limit, my position is clear," Merkel said in an interview broadcast live on Germany's ARD on Sunday. "I won't accept one."

Merkel will need the support of the Christian Social Union -- the more conservative sister party to Merkel's Christian Democratic Union -- in order to remain Chancellor after September elections.

On Monday, CSU leader Horst Seehofer reiterated his party's pledge to make the limit on refugees a reality.

"The cap is and remains a goal of the CSU," Seehofer said, although he refused to make the issue a red line in a coalition deal -- a softening of his previous stance.

In an interview in December, Horst Seehofer, head of the CSU, promised a cap of 200,000 refugees per year if his party is in a coalition government after the election.

'Division of labor'

It's the second public disagreement between the two parties in less than a month. Merkel abandoned her long-held position against same-sex marriage in June, opening the door for a vote on the issue in the German parliament, which passed just a few days later.

Some CSU politicians responded angrily. Hans Reichhart, member of the regional parliament in Bavaria, southern Germany, told Deutsche Welle that the CDU and CSU had made an agreement not to act on the issue during this parliament, which lasts until the election in September. "This agreement was broken," he said.

German parliament voted to legalize "Ehe fur alle" in June. Merkel voted against the bill.

Carsten Koschmieder, political scientist at the Freie Universitat in Berlin, sees these spats as part of the parties' campaign strategy rather than signs of fundamental disagreement.

"We're seeing a good division of labor between the CDU and the CSU," he told CNN. While Merkel makes her appeals to more liberal voters, the CSU can take the more conservative position on these controversial issues and appeal to a different group of voters.

The CSU can market itself as the "conservative corrective" to the more liberal CDU, he said.

And the parties do agree on the basic principles behind these issues. Both want to preserve traditional family structures and to reduce the number of refugees coming to Germany.

After rejecting the idea of a cap, Merkel highlighted the common ground she shares with the CSU. Both parties want to reduce the numbers and to "fight the causes of migration," she said. "And I believe we will achieve what we want. And without any kind of cap."

As long as the number of asylum seekers entering Germany remains relatively low, there will be no major clash between Merkel and her sister party, Koschmieder explained.

"But if Italy opens its borders and allows refugees to travel north through Europe, then we will have to see how Merkel and the CSU react," he said.

A 'useless debate'?

survey conducted last September found that 47% of those polled were in favor of an upper limit on the number of refugees, including 69% of CSU supporters.

Refugees and integration continues to top the list of concerns among German voters. Nearly half of Germans surveyed on July 7 in a weekly poll by Forschungsgruppe Wahlen said that the issue is among the most important for them, with the next most common issue (social inequality) mentioned by just 14% of respondents.

Some Germans have expressed their frustration with Merkel's latest comments on social media.

"Whoever votes for the CDU with Merkel is supporting further flooding of Germany," wrote Twitter user Anny's World.

Nach Merkels Interview gestern gibt es bei Ihr keine Obergrenze.Wer weiter CDU mit Merkel wählt unterstützt die weitere Überflutung D.lan — 💙Anny,s World 💙 (@ICHundS04) July 17, 2017

For others, a cap doesn't go far enough. "No upper limit," wrote Gerd Luck. "If there was an upper limit for refugees in Germany, it wouldn't solve the problem."

Keine Obergrenze Wenn es in Dtl eine Obergrenze für Flüchtlinge geben würde, ist das Problem auch nicht gelöst. Wir schaffen das nicht!!!! — Gerd Luck (@luck_gerd) July 17, 2017

But for Karl Kopp, director for European affairs at Pro Asyl, a German charity that advocates for refugees, any discussion of a cap is "a useless debate."

International law prohibits any kind of quota, Kopp explained. "Every asylum seeker has the right to have his claim addressed."

And he's convinced that the ongoing debate about the number of refugees is disguising the real problems facing refugees in Germany and across Europe.

Hundreds of refugees and migrants have died this year trying to reach Europe and many thousands have been rescued.

"There are 2,000 refugees in Greece living in desperate situations waiting for family reunification to Germany," Kopp said. "But the government has created a lot of obstacles and is trying to slow the process down. People wait there for years to join their family in Germany."

Meanwhile, hundreds of people are drowning in the Mediterranean as they try to reach Europe.

"We're losing a lot of time and energy talking about a quota that is not in line with international law," he said. What we need instead are "a new common European rescue approach" and "legal, safe ways for refugees to come to Europe."