BERLIN, April 11 (Reuters) - Almost half of Germany's first-time voters back Angela Merkel, a poll by the Forsa Institut showed on Tuesday, providing a strong backbone of support for the chancellor as she prepares to bid for a fourth term in office in September. Among all potential voters, Merkel had 43 percent support, compared to 32 percent for Martin Schulz, the chancellor candidate for the centre-left Social Democrats (SPD). But that lead extended to 47 percent against 29 percent among young voters aged 18 to 21, the poll showed. "Young people know Chancellor Merkel, with whom they grew up, but not the candidate Schulz," said Manfred Guellner, who heads the Forsa institute. He said the latest data showed that "especially young people are looking for stability and continuity in these uncertain times." The elder stateswoman of western European politics, Merkel has come under fire at home for initially opening Germany's doors to more than one million refugees. Facing what is likely to be a close-fought ballot, she has toughened her stance on immigration in recent months. Measured by party, the poll - conducted for Stern magazine and broadcaster RTL - put Merkel's Christian Democrats (CDU) and their Bavarian sister party (CSU) at 36 percent, 6 percentage points ahead of the SPD of former European Parliament president Schulz. A second survey by INSA for the mass-circulation Bild newspaper gave the CDU/CSU a 1.5 point lead ahead of the Sept. 24 national election. The anti-immigrant Alternative for Germany (AfD) party, which had seen its support weaken in recent months, was steady at 8 percent in the Forsa poll, adding 1 point to 10 percent in the INSA poll. The pro-business Free Democratic Party remains likely to return to parliament, gaining one percentage point to 6 percent in the Forsa poll and 1.5 percentage points to 6.5 percent in the INSA poll. Both Merkel and Schulz are hoping to form new governments with smaller parties, but the two polls suggested the "grand coalition" the two parties currently operate is likely to continue.