BERLIN (Reuters) - German Chancellor Angela Merkel's conservatives would win more votes than the center-left Social Democrats (SPD) if an election were to be held this Sunday, a survey showed, six months before a national vote in Europe's political and economic powerhouse.


It also suggested that another grand coalition of conservatives and the SPD was the most likely outcome of the September election.


The conservatives gained one percentage point to 34 percent in the GMS poll published on Thursday compared with the same survey at the start of February. Support for the SPD increased by 2 percentage points to 31 percent.


The SPD has shot up in opinion polls since nominating former European Parliament president Martin Schulz in late January to challenge Merkel. It has also attracted thousands of new members since then as Schulz focuses his campaign on social justice.


But GMS said the upside from Schulz's nomination seemed to have been exhausted: "The Schulz effect has come to a halt for the time being: While the SPD is on 31 percent, it's behind the conservatives."

The far-left Linke and Greens were both on 8 percent.

Some polls have suggested that a left-leaning 'red-red-green' alliance of the SPD, Linke and Greens could oust Merkel and replace the current 'grand coalition' of conservatives and SPD after the Sept. 24 national election.

But GMS said its survey suggested that was unlikely.

"Despite the SPD making significant gains after nominating Martin Schulz as its chancellor candidate, there are currently still no realistic coalition options with a mathematical government majority other than another grand coalition," GMS said in a statement.

It said this was because the SPD's gains in the last two months had largely been at the expense of the Greens and the Linke.

The right-wing Alternative for Germany (AfD) looked set to enter the national parliament for the first time with 9 percent of votes but that was its worst result in this survey for more than half a year as it lost some support to the SPD.

The pro-business Free Democrats (FDP) were on 6 percent, meaning they also look set to clear the 5 percent hurdle to return to the national assembly after crashing out of parliament in the 2013 election.

The survey of 1,008 people was carried out from March 16 to 22.

(Reporting by Michelle Martin; Editing by Michael Nienaber/Jeremy Gaunt)