By Paul Carrel and Joseph Nasr
BERLIN (Reuters) - Chancellor Angela Merkel's junior coalition partners gave her conservatives until next year to deliver more policy results, threatening to end their alliance if there is no improvement after both parties suffered in a regional election on Sunday.
Merkel's conservative Christian Democrats (CDU) came home first in the election in the western state of Hesse, but polled just 28 percent of the vote, an exit poll for broadcaster ARD showed. That marked a huge drop from the 38.3 percent the CDU won at the last Hesse election, in 2013.
The centre-left Social Democrats (SPD) fared even worse, winning just 20 percent of the vote, down from 30.7 percent and its worst result in the western state since 1946. The party just managed to beat the Greens, who placed third with 19.5 percent.
SPD leader Andrea Nahles said she would use a roadmap with which to measure the progress of the ruling coalition, which has been plagued by infighting, at a mid-term review next year.
"We could then gauge the implementation of this roadmap at the agreed mid-term review, when we would be able to clearly see if this government is the right place for us," Nahles told reporters. "The state of the government is unacceptable."
Her message was clear: the SPD needs to be able to show tangible results to its supporters next year or else the party's leaders will pull out of the coalition with Merkel.
Volker Bouffier, the incumbent CDU state premier in Hesse and a Merkel ally, said his party had achieved its goal of being able to lead the next government in Hesse, but added: "We are in pain because of the losses".
"The message to the parties ruling in Berlin is: People want fewer disputes and more focus on the important issues," he said.
The CDU's poor result in Hesse, after its sister party in the state of Bavaria, the CSU, suffered its worst result there since 1950 two weeks ago, may turbo-charge a debate about who succeeds Merkel and when. She has been chancellor for 13 years.
Merkel's weakness at home may limit her capacity to lead in the European Union at a time when the bloc is dealing with Brexit, a budget crisis in Italy and the prospect of populist parties making gains at European parliament elections next May.
The Greens' strong performance in Hesse means Bouffier will likely be able to remain state premier at the helm of a CDU/Greens government.
The other big winner was the far-right Alternative for Germany (AfD), which entered the Hesse regional assembly for the first time with 12 percent of the vote, the ARD exit poll showed.
The result means the anti-immigration party, which entered the federal parliament for the first time last year, is now also represented in all 16 German regional assemblies.
Merkel's ruling coalition "has lost the confidence of the electorate", said Josef Joffe, publisher-editor of weekly Die Zeit.
Of the SPD, he added: "A party on the way down cannot suddenly rise from the ashes by going into the opposition. So the party grandees will clench their teeth, stay in the coalition and wait for a better day."
Merkel's fourth and probably final government has already come close to collapsing twice. Nahles' comments show the SPD will put more pressure on the conservatives to deliver policy results for the centre-left party.
Merkel's conservatives only formed their loveless national partnership with the SPD in March after the collapse of talks on a three-way coalition of the conservatives, Greens and pro-business FDP.
The ARD exit poll showed only 13 percent of CDU voters believed Merkel had helped the party in Hesse, down from 70 percent at the last state election, reflecting voter anger at her decision in 2015 to welcome almost one million, mainly Muslim asylum seekers.
The CDU holds its annual congress in December, when Merkel will seek re-election as party chairwoman. She is likely to be reappointed but a weak show of support for her would undermine her authority and accelerate the succession debate.
(Reporting by Paul Carrel; Editing by Matthew Mpoke Bigg)