BERLIN (Reuters) - German Chancellor Angela Merkel's conservative Christian Democrats (CDU) and their Christian Social Union (CSU) sister party moved back in front of the center-left Social Democrats (SPD) in an INSA poll for Bild newspaper published on Monday.


The INSA poll of 2,030 voters between Feb. 17 and 20 showed the CDU/CSU would win 31.5 percent, up 1.5 percentage points in the last week, while the resurgent SPD would win 30 percent, down 1 point. An INSA poll on Feb. 6 showed the long-suffering SPD ahead of the CDU/CSU for the first time in many years.


On Sunday, a poll by the leading Emnid institute found the SPD ahead of the CDU/CSU for the first time since 2006, boosted by the appointment of Martin Schulz as the chancellor candidate who wants to take his party to the left to beat Merkel.


The SPD's unexpected surge of some 12 points in the last month has caught Merkel and her conservatives off guard, analysts have said, just seven months before the Sept. 24 election, where she had expected to win a fourth term easily.


The Emnid poll of 1,885 voters gave the SPD 33 percent of the vote, up 1 point in the last week, while the CDU/CSU would win 32 percent, down 1 point.


The INSA poll also had the SPD's possible left-leaning coalition partners slipping in the last week. The hard-left Linke were down 0.5 point to 9.5 percent and the Greens were down 0.5 point to 6.5 percent.

The CDU/CSU's preferred coalition partner, the Free Democrats (FDP), were up 0.5 point to 5.5 percent. The ostracized far-right Alternative for Germany (AfD) were down 1 point to 11 percent, INSA said.

The SPD, junior partner in Merkel's ruling coalition, had trailed her conservative bloc for years in opinion polls until the nomination of Schulz revived the party. It last won an election under Gerhard Schroeder in 2002.

On Friday, the Electoral Research Group published an opinion poll for ZDF television showing the SPD at 30 percent but still behind the CDU/CSU, which was at 34 percent.

(Reporting by Erik Kirschbaum; Editing by Jonathan Oatis)