As the unpopular Westerwelle, who is vice chancellor and leader of the pro-business Free Democratic Party (FDP) prepares for a make-or-break party conference this week, Seehofer warned on Wednesday that the existence of the centre-right government was at stake.
“I hope that the FDP stabilises,” said Seehofer, who leads the Christian Social Union – the Bavarian sister party to Angela Merkel's Christian Democratic Union.
He told daily Süddeutsche Zeitung that the issue of Westerwelle's crippling unpopularity was “plainly an issue for the coalition.”
“Christian-liberal or red-green; that is the alternative,” said Seehofer, referring to voters' choice between the existing centre-right coalition or an alliance of the centre-left Social Democratic Party and environmentalist Greens.
The FDP's dramatic poll slump to just 3 percent approval has sparked calls within the party for Westerwelle's resignation, though senior party members are publicly backing him for now. He faces a tough party conference at which he will give a key speech on Thursday – generally regarded as a make-or-break speech that will need to impress followers.
The pro-market but socially liberal FDP has frequently clashed with the conservative, more traditionalist CSU, causing headaches for Merkel's coalition government.
However, Seehofer backed Westerwelle in his comments to Süddeutsche Zeitung.
“To me, he belongs to the very strongest figures of the German liberals,” Seehofer said.