(Adds comment from SPD leader Martin Schulz)
BERLIN, Sept 21 (Reuters) - Fears that apathy could boost the far-right Alternative for Germany (AfD) in an election on Sunday have driven Chancellor Angela Merkel and her main rival to urge their backers to go out and vote.
With many voters viewing a fourth term for Merkel as almost inevitable and put off by a turgid campaign - occasionally punctured by heckling and tomato hurling in protest at Merkel's refugee policy - pollsters say turnout may be low.
A GMS poll on Thursday said 34 percent were not planning to vote or were undecided, higher than the 29 percent who did not cast ballots in the last election in 2013.
"My request to everyone is that they vote, and vote for those parties that adhere 100 percent to our constitution," Merkel told MDR radio on Thursday, pointing to the AfD.
The party, running at between nine and 12 percent in opinion polls, has been likened by Foreign Minister Sigmar Gabriel and some commentators to Hitler's Nazis.
Some of its members have called for Germans to rewrite the history books on the Nazi era and the party has been dogged by rows over Holocaust denial, a crime in Germany.
Polls show conservative Merkel, trusted by stability-loving Germans to stand up to unpredictable leaders in the likes of the United States, Russia and Turkey, is on track to win.
But the shape of her coalition is wide open.
Most pollsters say a low turnout could boost the AfD, which has gained popularity by focusing on migration and security.
While mainstream parties have ruled out including the AfD in any governing coalition, it is poised to be the first far-right party to enter Germany's parliament in over half a century.
Gabriel told Russian broadcaster RT this week that not all AfD members or voters were Nazis, but the party has "people at the helm of the AfD that are inciting people to hatred, that are trading in Nazi propaganda."
Comments by some of the party's top members have caused outrage, including one leader who this month said Germany should be proud of its soldiers' achievements in World War Two.
'WILLINGNESS TO VOTE'
Allensbach pollster Renate Koecher told Die Zeit weekly that much depended on turnout. "Willingness to vote" was strongest among conservative and AfD supporters, she said. Most undecideds were FDP, Green and Left voters, indicating that a higher turnout would help the smaller parties on the left, she said.
The two biggest blocs have dipped slightly in most polls in the last month or so while some of the smaller parties have inched up.
In the GMS survey, the AfD was up two points at 10 percent.
Merkel's conservative bloc slipped one percentage point to 37 percent and her main opponent, the Social Democrats (SPD), were unchanged on 22 percent. SPD leader Martin Schulz has also urged his supporters to vote, saying "everything is still possible."
In a highly unusual intervention this week, Merkel's right-hand man Peter Altmaier, head of the chancellor's office, stirred controversy by suggesting it would be better for Germans not to vote at all rather than vote for the AfD.
Schulz told broadcaster N24 it was not sensible to encourage people not to vote, adding: "I urge AfD voters to consider the following: Whoever wants social justice, respect in our society and above all for Angela Merkel to be replaced must make the SPD strong."
SPD Justice Minister Heiko Maas accused Altmaier of helping the AfD. "Telling people not to vote is ...exactly what they want," he said.
Latest surveys suggest coalition options are limited to another grand coalition of Merkel's conservatives with the SPD, or a three-way "Jamaica" alliance of the conservatives, FDP and Greens which could limit Merkel's room for maneuver on euro zone reform.
Both the Greens and FDP - who are at opposite ends of the political spectrum - have played down the likelihood of joining forces, fearing it would put off their voters. But on Thursday, FDP leader Christian Lindner struck a more positive note.
"The FDP is enthusiastic about going into government. But only if it can implement good things. If not, then we won't," he told Die Welt.
(Additional reporting by Thomas Escritt and Michelle Martin; Editing by Toby Chopra and John Stonestreet)